Incremental fsck
Subsystems (2)
Quoted 10 lines · Andi Kleen
Quoted 3 lines · Theodore Tso
Now, there are good reasons for doing periodic checks every N mounts and after M months. And it has to do with PC class hardware. (Ted's
aphorism: "PC class hardware is cr*p").If these reasons are good ones (some skepticism here) then the correct way to really handle this would be to do regular background scrubbing during runtime; ideally with metadata checksums so that you can actually detect all corruption.
Quoted 5 lines
But since fsck is so slow and disks are so big this whole thing is a ticking time bomb now. e.g. it is not uncommon to require tens of minutes or even hours of fsck time and some server that reboots only every few months will eat that when it happens to reboot. This means you get a quite long downtime.
Has there been some thought about an incremental fsck?
You know, somehow fencing a sub-dir to do an online fsck?
Thanks for some thoughts!
--
Al
Quoted 3 lines · On Wed, 9 Jan 2008 00:22:55 +0300 Al Boldi <[email protected]> wrote:
Has there been some thought about an incremental fsck?
You know, somehow fencing a sub-dir to do an online fsck?
Search for "chunkfs"
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Quoted 6 lines · Rik van Riel
Quoted 3 lines · Al Boldi
Has there been some thought about an incremental fsck?
You know, somehow fencing a sub-dir to do an online fsck?
Search for "chunkfs"
Sure, and there is TileFS too.
But why wouldn't it be possible to do this on the current fs infrastructure, using just a smart fsck, working incrementally on some sub-dir?
Thanks!
--
Al
Quoted 4 lines · Al Boldi
Quoted 3 lines · Rik van Riel
Quoted 2 lines · Al Boldi
Has there been some thought about an incremental fsck?
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Quoted 1 line
You know, somehow fencing a sub-dir to do an online fsck?
Search for "chunkfs"
Sure, and there is TileFS too.
But why wouldn't it be possible to do this on the current fs infrastructure, using just a smart fsck, working incrementally on some sub-dir?
Several data structures are file system wide and require finding every allocated file and block to check that they are correct. In particular, block and inode bitmaps can't be checked per subdirectory.
http://infohost.nmt.edu/~val/review/chunkfs.pdf
-VAL
-VAL
Quoted 11 lines · Valerie Henson
Quoted 10 lines · Al Boldi
Quoted 6 lines · Rik van Riel
Quoted 3 lines · Al Boldi
Has there been some thought about an incremental fsck?
You know, somehow fencing a sub-dir to do an online fsck?
Search for "chunkfs"
Sure, and there is TileFS too.
Quoted 7 lines
Quoted 3 lines
But why wouldn't it be possible to do this on the current fs infrastructure, using just a smart fsck, working incrementally on some sub-dir?
Several data structures are file system wide and require finding every allocated file and block to check that they are correct. In particular, block and inode bitmaps can't be checked per subdirectory.
Ok, but let's look at this a bit more opportunistic / optimistic.
Even after a black-out shutdown, the corruption is pretty minimal, using ext3fs at least. So let's take advantage of this fact and do an optimistic fsck, to assure integrity per-dir, and assume no external corruption. Then we release this checked dir to the wild (optionally ro), and check the next. Once we find external inconsistencies we either fix it unconditionally, based on some preconfigured actions, or present the user with options.
All this could be per-dir or using some form of on-the-fly file-block-zoning.
And there probably is a lot more to it, but it should conceptually be possible, with more thoughts though...
Quoted 1 line
Thanks!
--
Al
Quoted 1 line · On Wed, 9 Jan 2008 14:52:14 +0300 Al Boldi <[email protected]> wrote:
Ok, but let's look at this a bit more opportunistic / optimistic.
You can't play fast and loose with data integrity.
Besides, if we looked at things optimistically, we would conclude that no fsck will be needed, ever :)
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You will really want to read this paper, if you haven't already.
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All Rights Reversed
Quoted 4 lines · Rik van Riel
Quoted 1 line · Al Boldi
Ok, but let's look at this a bit more opportunistic / optimistic.
You can't play fast and loose with data integrity.
Correct, but you have to be realistic...
Quoted 2 lines
Besides, if we looked at things optimistically, we would conclude that no fsck will be needed,
And that's the reality, because people are mostly optimistic and feel extremely tempted to just force-mount a dirty ext3fs, instead of waiting hours-on-end for a complete fsck, which mostly comes back with some benign "inode should be zero" warning.
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ever :)
Well not ever, but most people probably fsck during scheduled shutdowns, or when they are forced to, due to online fs accessibility errors.
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You will really want to read this paper, if you haven't already.
Definitely a good read, but attacking the problem from a completely different POV.
BTW: Dropped some cc's due to bounces.
Thanks!
--
Al
Quoted 6 lines · Al Boldi
Ok, but let's look at this a bit more opportunistic / optimistic.
Even after a black-out shutdown, the corruption is pretty minimal, using ext3fs at least.
After a unclean shutdown, assuming you have decent hardware that doesn't lie about when blocks hit iron oxide, you shouldn't have any corruption at all. If you have crappy hardware, then all bets are off....
Quoted 6 lines
So let's take advantage of this fact and do an optimistic fsck, to assure integrity per-dir, and assume no external corruption. Then we release this checked dir to the wild (optionally ro), and check the next. Once we find external inconsistencies we either fix it unconditionally, based on some preconfigured actions, or present the user with options.
So what can you check? The *only* thing you can check is whether or not the directory syntax looks sane, whether the inode structure looks sane, and whether or not the blocks reported as belong to an inode looks sane.
What is very hard to check is whether or not the link count on the inode is correct. Suppose the link count is 1, but there are actually two directory entries pointing at it. Now when someone unlinks the file through one of the directory hard entries, the link count will go to zero, and the blocks will start to get reused, even though the inode is still accessible via another pathname. Oops. Data Loss.
This is why doing incremental, on-line fsck'ing is *hard*. You're not going to find this while doing each directory one at a time, and if the filesystem is changing out from under you, it gets worse. And it's not just the hard link count. There is a similar issue with the block allocation bitmap. Detecting the case where two files are simultaneously can't be done if you are doing it incrementally, and if the filesystem is changing out from under you, it's impossible, unless you also have the filesystem telling you every single change while it is happening, and you keep an insane amount of bookkeeping.
One that you *might* be able to do, is to mount a filesystem readonly, check it in the background while you allow users to access it read-only. There are a few caveats, however ---- (1) some filesystem errors may cause the data to be corrupt, or in the worst case, could cause the system to panic (that's would arguably be a filesystem/kernel bug, but we've not necessarily done as much testing here as we should.) (2) if there were any filesystem errors found, you would beed to completely unmount the filesystem to flush the inode cache and remount it before it would be safe to remount the filesystem read/write. You can't just do a "mount -o remount" if the filesystem was modified under the OS's nose.
Quoted 4 lines
All this could be per-dir or using some form of on-the-fly file-block-zoning.
And there probably is a lot more to it, but it should conceptually be possible, with more thoughts though...
Many things are possible, in the NASA sense of "with enough thrust, anything will fly". Whether or not it is *useful* and *worthwhile* are of course different questions! :-)
- Ted
Quoted 9 lines · Theodore Tso
Quoted 4 lines · Al Boldi
Ok, but let's look at this a bit more opportunistic / optimistic.
Even after a black-out shutdown, the corruption is pretty minimal, using ext3fs at least.
After a unclean shutdown, assuming you have decent hardware that doesn't lie about when blocks hit iron oxide, you shouldn't have any corruption at all. If you have crappy hardware, then all bets are off....
Maybe with barriers...
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So let's take advantage of this fact and do an optimistic fsck, to assure integrity per-dir, and assume no external corruption. Then we release this checked dir to the wild (optionally ro), and check the next. Once we find external inconsistencies we either fix it unconditionally, based on some preconfigured actions, or present the user with options.
So what can you check? The *only* thing you can check is whether or not the directory syntax looks sane, whether the inode structure looks sane, and whether or not the blocks reported as belong to an inode looks sane.
Which would make this dir/area ready for read/write access.
Quoted 6 lines
What is very hard to check is whether or not the link count on the inode is correct. Suppose the link count is 1, but there are actually two directory entries pointing at it. Now when someone unlinks the file through one of the directory hard entries, the link count will go to zero, and the blocks will start to get reused, even though the inode is still accessible via another pathname. Oops. Data Loss.
We could buffer this, and only actually overwrite when we are completely finished with the fsck.
Quoted 9 lines
This is why doing incremental, on-line fsck'ing is *hard*. You're not going to find this while doing each directory one at a time, and if the filesystem is changing out from under you, it gets worse. And it's not just the hard link count. There is a similar issue with the block allocation bitmap. Detecting the case where two files are simultaneously can't be done if you are doing it incrementally, and if the filesystem is changing out from under you, it's impossible, unless you also have the filesystem telling you every single change while it is happening, and you keep an insane amount of bookkeeping.
Ok, you have a point, so how about we change the implementation detail a bit, from external fsck to internal fsck, leveraging the internal fs bookkeeping, while allowing immediate but controlled read/write access.
Thanks for more thoughts!
--
Al
Quoted 11 lines · Theodore Tso
Quoted 6 lines · Al Boldi
Ok, but let's look at this a bit more opportunistic / optimistic.
Even after a black-out shutdown, the corruption is pretty minimal, using ext3fs at least.
After a unclean shutdown, assuming you have decent hardware that doesn't lie about when blocks hit iron oxide, you shouldn't have any corruption at all. If you have crappy hardware, then all bets are off....
What hardware is crappy here. Lets say... internal hdd in thinkpad x60?
What are ext3 expectations of disk (is there doc somewhere)? For example... if disk does not lie, but powerfail during write damages the sector -- is ext3 still going to work properly?
If disk does not lie, but powerfail during write may cause random numbers to be returned on read -- can fsck handle that?
What abou disk that kills 5 sectors around sector being written during powerfail; can ext3 survive that?
Pavel
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Quoted 3 lines
What are ext3 expectations of disk (is there doc somewhere)? For example... if disk does not lie, but powerfail during write damages the sector -- is ext3 still going to work properly?
Nope. However the few disks that did this rapidly got firmware updates because there are other OS's that can't cope.
Quoted 2 lines
If disk does not lie, but powerfail during write may cause random numbers to be returned on read -- can fsck handle that?
most of the time. and fsck knows about writing sectors to remove read errors in metadata blocks.
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What abou disk that kills 5 sectors around sector being written during powerfail; can ext3 survive that?
generally. Note btw that for added fun there is nothing that guarantees the blocks around a block on the media are sequentially numbered. The usually are but you never know.
Alan
PATCH 1/1 document ext3 requirements (was Re: [RFD] Incremental fsck)
Hi!
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What are ext3 expectations of disk (is there doc somewhere)? For example... if disk does not lie, but powerfail during write damages the sector -- is ext3 still going to work properly?
Nope. However the few disks that did this rapidly got firmware updates because there are other OS's that can't cope.
Quoted 2 lines
If disk does not lie, but powerfail during write may cause random numbers to be returned on read -- can fsck handle that?
most of the time. and fsck knows about writing sectors to remove read errors in metadata blocks.
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What abou disk that kills 5 sectors around sector being written during powerfail; can ext3 survive that?
generally. Note btw that for added fun there is nothing that guarantees the blocks around a block on the media are sequentially numbered. The usually are but you never know.
Ok, should something like this be added to the documentation?
It would be cool to be able to include few examples (modern SATA disks support bariers so are safe, any IDE from 1989 is unsafe), but I do not know enough about hw...
Documentation/filesystems/ext3.txt +12 -0
@@ -183,6 +183,18 @@ mke2fs: create a ext3 partition with th debugfs: ext2 and ext3 file system debugger. ext2online: online (mounted) ext2 and ext3 filesystem resizer +Requirements +============ + +Ext3 needs disk that does not do write-back caching or disk that +supports barriers and Linux configuration that can use them. + +* if disk damages the sector being written during powerfail, ext3 + can't cope with that. Fortunately, such disks got firmware updates + to fix this long time ago. + +* if disk writes random data during powerfail, ext3 should survive + that most of the time. References ==========
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 22 lines · Pavel Machek
Hi!
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What are ext3 expectations of disk (is there doc somewhere)? For example... if disk does not lie, but powerfail during write damages the sector -- is ext3 still going to work properly?
Nope. However the few disks that did this rapidly got firmware updates because there are other OS's that can't cope.
Quoted 2 lines
If disk does not lie, but powerfail during write may cause random numbers to be returned on read -- can fsck handle that?
most of the time. and fsck knows about writing sectors to remove read errors in metadata blocks.
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What abou disk that kills 5 sectors around sector being written during powerfail; can ext3 survive that?
generally. Note btw that for added fun there is nothing that guarantees the blocks around a block on the media are sequentially numbered. The usually are but you never know.
Quoted 5 lines
Ok, should something like this be added to the documentation?
It would be cool to be able to include few examples (modern SATA disks support bariers so are safe, any IDE from 1989 is unsafe), but I do not know enough about hw...
ext3 is not the only filesystem that will have trouble due to volatile write caches. We see problems often enough with XFS due to volatile write caches that it's in our FAQ:
http://oss.sgi.com/projects/xfs/faq.html#wcache
Cheers,
Dave.
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Dave Chinner Principal Engineer SGI Australian Software Group
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Hi!
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What are ext3 expectations of disk (is there doc somewhere)? For example... if disk does not lie, but powerfail during write damages the sector -- is ext3 still going to work properly?
Nope. However the few disks that did this rapidly got firmware updates because there are other OS's that can't cope.
Quoted 2 lines
If disk does not lie, but powerfail during write may cause random numbers to be returned on read -- can fsck handle that?
most of the time. and fsck knows about writing sectors to remove read errors in metadata blocks.
Quoted 6 lines
Quoted 2 lines
What abou disk that kills 5 sectors around sector being written during powerfail; can ext3 survive that?
generally. Note btw that for added fun there is nothing that guarantees the blocks around a block on the media are sequentially numbered. The usually are but you never know.
Quoted 5 lines
Ok, should something like this be added to the documentation?
It would be cool to be able to include few examples (modern SATA disks support bariers so are safe, any IDE from 1989 is unsafe), but I do not know enough about hw...
Quoted 5 lines
ext3 is not the only filesystem that will have trouble due to volatile write caches. We see problems often enough with XFS due to volatile write caches that it's in our FAQ:
Nice FAQ, yep. Perhaps you should move parts of it to Documentation/ , and I could then make ext3 FAQ point to it?
I had write cache enabled on my main computer. Oops. I guess that means we do need better documentation.
Pavel
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 2 lines · Pavel Machek
I had write cache enabled on my main computer. Oops. I guess that means we do need better documentation.
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
Regards,
Daniel
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 5 lines
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
AFAIK no drive saves the cache. The worst case cache flush for drives is several seconds with no retries and a couple of minutes if something really bad happens.
This is why the kernel has some knowledge of barriers and uses them to issue flushes when needed.
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 6 lines · Alan Cox
Quoted 5 lines
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
Quoted 6 lines
AFAIK no drive saves the cache. The worst case cache flush for drives is several seconds with no retries and a couple of minutes if something really bad happens.
This is why the kernel has some knowledge of barriers and uses them to issue flushes when needed.
Indeed, you are right, which is supported by actual measurements:
http://sr5tech.com/write_back_cache_experiments.htm
Sorry for implying that anybody has engineered a drive that can do such a nice thing with writeback cache.
The "disk motor as a generator" tale may not be purely folklore. When an IDE drive is not in writeback mode, something special needs to done to ensure the last write to media is not a scribble.
A small UPS can make writeback mode actually reliable, provided the system is smart enough to take the drives out of writeback mode when the line power is off.
Regards,
Daniel
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 22 lines · On Tue, 15 Jan 2008 20:24:27 -0500 "Daniel Phillips" <[email protected]>…
Quoted 7 lines · Alan Cox
Quoted 6 lines
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
Quoted 6 lines
AFAIK no drive saves the cache. The worst case cache flush for drives is several seconds with no retries and a couple of minutes if something really bad happens.
This is why the kernel has some knowledge of barriers and uses them to issue flushes when needed.
Indeed, you are right, which is supported by actual measurements:
http://sr5tech.com/write_back_cache_experiments.htm
Sorry for implying that anybody has engineered a drive that can do such a nice thing with writeback cache.
Quoted 7 lines
The "disk motor as a generator" tale may not be purely folklore. When an IDE drive is not in writeback mode, something special needs to done to ensure the last write to media is not a scribble.
A small UPS can make writeback mode actually reliable, provided the system is smart enough to take the drives out of writeback mode when the line power is off.
We've had mount -o barrier=1 for ext3 for a while now, it makes writeback caching safe. XFS has this on by default, as does reiserfs.
-chris
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 29 lines · Chris Mason
Quoted 26 lines · On Tue, 15 Jan 2008 20:24:27 -0500 "Daniel Phillips" <[email protected]>…
Quoted 7 lines · Alan Cox
Quoted 6 lines
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
Quoted 6 lines
AFAIK no drive saves the cache. The worst case cache flush for drives is several seconds with no retries and a couple of minutes if something really bad happens.
This is why the kernel has some knowledge of barriers and uses them to issue flushes when needed.
Indeed, you are right, which is supported by actual measurements:
http://sr5tech.com/write_back_cache_experiments.htm
Sorry for implying that anybody has engineered a drive that can do such a nice thing with writeback cache.
The "disk motor as a generator" tale may not be purely folklore. When an IDE drive is not in writeback mode, something special needs to done to ensure the last write to media is not a scribble.
Quoted 6 lines
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A small UPS can make writeback mode actually reliable, provided the system is smart enough to take the drives out of writeback mode when the line power is off.
We've had mount -o barrier=1 for ext3 for a while now, it makes writeback caching safe. XFS has this on by default, as does reiserfs.
Maybe ext3 should do barriers by default? Having ext3 in "lets corrupt data by default"... seems like bad idea.
Pavel
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Daniel PhillipsQuoted 3 lines
The "disk motor as a generator" tale may not be purely folklore. When an IDE drive is not in writeback mode, something special needs to done to ensure the last write to media is not a scribble.
No it doesn't. The last write _is_ a scribble. Systems that make atomic updates to disk drives use a shadow update mechanism and write the master sector twice. If the power fails in the middle of writing one, it will almost certainly be unreadable due to a CRC failure, and the other one will have either the old or new master block contents.
And I think there's a problem with drives that, upon sensing the unreadable sector, assign an alternate even though the sector is fine, and you eventually run out of spares.
Incidentally, while this primitive behavior applies to IDE (ATA et al) drives, that isn't the only thing people put filesystem on. Many important filesystems go on higher level storage subsystems that contain IDE drives and cache memory and batteries. A device like this _does_ make sure that all data that it says has been written is actually retrievable even if there's a subsequent power outage, even while giving the performance of writeback caching.
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 3 lines
And I think there's a problem with drives that, upon sensing the unreadable sector, assign an alternate even though the sector is fine, and you eventually run out of spares.
You are assuming drives can't tell the difference between stray data loss and sectors that can't be recovered by rewriting and reuse. I was under the impression modern drives could do this ?
Alan
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Bryan HendersonQuoted 5 lines · Bryan Henderson
Quoted 3 lines
The "disk motor as a generator" tale may not be purely folklore. When an IDE drive is not in writeback mode, something special needs to done to ensure the last write to media is not a scribble.
No it doesn't. The last write _is_ a scribble.
Have you observed that in the wild? A former engineer of a disk drive company suggests to me that the capacitors on the board provide enough power to complete the last sector, even to park the head.
Regards,
Daniel
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
"Daniel Phillips" <[email protected]> wrote on 01/16/2008 06:02:50 PM:
Quoted 3 lines · Bryan Henderson
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The "disk motor as a generator" tale may not be purely folklore. When an IDE drive is not in writeback mode, something special needs to
done
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to ensure the last write to media is not a scribble.
No it doesn't. The last write _is_ a scribble.
Have you observed that in the wild? A former engineer of a disk drive company suggests to me that the capacitors on the board provide enough power to complete the last sector, even to park the head.
No, I haven't. It's hearsay, and from about 3 years ago.
As for parking the head, that's hard to believe, since it's so easy and more reliable to use a spring and an electromagnet.
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Daniel PhillipsQuoted 5 lines · Daniel Phillips
Have you observed that in the wild? A former engineer of a disk drive company suggests to me that the capacitors on the board provide enough power to complete the last sector, even to park the head.
The problem isn't with the disk drive; it's from the DRAM, which tend to be much more voltage sensitive than the hard drives --- so it's quite likely that you could end up DMA'ing garbage from the memory. In fact the fact that the disk drives lasts longer due to capacitors on the board, rotational inertia of the platters, etc., is part of the problem.
It was observed in the wild by SGI, many years ago on their hardware. They later added extra capacitors on the motherboard and a powerfail interrupt which caused the Irix to run around frantically shutting down DMA's for a controlled shutdown. Of course, PC-class hardware has none of this. My source for this was Jim Mostek, one of the original Linux XFS porters. He had given me source code to a test program that would show this; basically zeroed out a region of disk, then started writing series of patterns on that part of the, and you you kicked out the power cord, and then see if there was any garbage on the disk. If you saw something that wasn't one of the patterns being written to the disk, then you knew you had a problem. I can't find the program any more, but it wouldn't be hard to write.
I do know that I have seen reports from many ext2 users in the field that could only be explained by the hard drive scribbling garbage onto the inode table. Ext3 solves this problem because of its physical block journaling.
- Ted
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 4 lines
interrupt which caused the Irix to run around frantically shutting down DMA's for a controlled shutdown. Of course, PC-class hardware has none of this. My source for this was Jim Mostek, one of the
PC class hardware has a power good signal which drops just before the rest.
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Theodore TsoQuoted 5 lines · Theodore Tso
Quoted 4 lines · Daniel Phillips
Have you observed that in the wild? A former engineer of a disk drive company suggests to me that the capacitors on the board provide enough power to complete the last sector, even to park the head.
Even if true (which I doubt), this is not implemented.
A modern drive can have 16-32 MB of write cache. Worst case, those sectors are not sequential which implies lots of head movement.
Quoted 7 lines
The problem isn't with the disk drive; it's from the DRAM, which tend to be much more voltage sensitive than the hard drives --- so it's quite likely that you could end up DMA'ing garbage from the memory. In fact the fact that the disk drives lasts longer due to capacitors on the board, rotational inertia of the platters, etc., is part of the problem.
I can tell you directly that when you drop power to a drive, you will lose write cache data if the write cache is enabled. With barriers enabled, our testing shows that file systems survive power failures which routinely caused corruption without them ;-)
ric
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Ric Wheeler <[email protected]> wrote on 01/17/2008 03:18:05 PM:
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Have you observed that in the wild? A former engineer of a disk
drive
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company suggests to me that the capacitors on the board provide
enough
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power to complete the last sector, even to park the head.
Even if true (which I doubt), this is not implemented.
A modern drive can have 16-32 MB of write cache. Worst case, those sectors are not sequential which implies lots of head movement.
We weren't actually talking about writing out the cache. While that was part of an earlier thread which ultimately conceded that disk drives most probably do not use the spinning disk energy to write out the cache, the claim was then made that the drive at least survives long enough to finish writing the sector it was writing, thereby maintaining the integrity of the data at the drive level. People often say that a disk drive guarantees atomic writes at the sector level even in the face of a power failure.
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Ted brought up the separate issue of the host sending garbage to the disk device because its own power is failing at the same time, which makes the integrity at the disk level moot (or even undesirable, as you'd rather write a bad sector than a good one with the wrong data).
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 4 lines · Bryan Henderson
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Well, it would be impossible or at least very hard to see that in practice, right? My understanding is that drives do sector-level checksums, so if there was a partially written sector, the checksum would be bogus and the drive would return an error when you tried to read from it.
Quoted 4 lines
Ted brought up the separate issue of the host sending garbage to the disk device because its own power is failing at the same time, which makes the integrity at the disk level moot (or even undesirable, as you'd rather write a bad sector than a good one with the wrong data).
Yep, exactly. It would be interesting to see if this happens on modern hardware; all of the evidence I've had for this is years old at this point.
- Ted
Re: [Patch] document ext3 requirements (was Re: [RFD] Incrementalfsck)
Quoted 17 lines · Theodore Tso
Quoted 4 lines · Bryan Henderson
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Well, it would be impossible or at least very hard to see that in practice, right? My understanding is that drives do sector-level checksums, so if there was a partially written sector, the checksum would be bogus and the drive would return an error when you tried to read from it.
Quoted 4 lines
Ted brought up the separate issue of the host sending garbage to the disk device because its own power is failing at the same time, which makes the integrity at the disk level moot (or even undesirable, as you'd rather write a bad sector than a good one with the wrong data).
Quoted 5 lines
Yep, exactly. It would be interesting to see if this happens on modern hardware; all of the evidence I've had for this is years old at this point.
- Ted
I have a Seagate Barracuda 7200.9 80 Gbyte SATA drive that I use for experiments. I can permanently destroy a EXT3 file-system at least 50% of the time by disconnecting the data cable while a dd write to a file is in progress. Something bad happens making partition information invalid. I have to re-partition to reuse the drive.
If I try the same experiment by disconnecting power to the drive the file is no good (naturally), but the rest of the file-system is fine.
My theory is that the destination offset is present in every SATA access and some optimization code within the drive sets the heads to track zero and writes before any CRC or checksum is done to find out if it was the correct offset with the correct data!
Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.29 BogoMips). My book : http://www.AbominableFirebug.com/
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incrementalfsck)
Quoted 24 lines · linux-os (Dick Johnson)
Quoted 12 lines · Theodore Tso
Quoted 4 lines · Bryan Henderson
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Well, it would be impossible or at least very hard to see that in practice, right? My understanding is that drives do sector-level checksums, so if there was a partially written sector, the checksum would be bogus and the drive would return an error when you tried to read from it.
Quoted 8 lines
Quoted 4 lines
Ted brought up the separate issue of the host sending garbage to the disk device because its own power is failing at the same time, which makes the integrity at the disk level moot (or even undesirable, as you'd rather write a bad sector than a good one with the wrong data).
Yep, exactly. It would be interesting to see if this happens on modern hardware; all of the evidence I've had for this is years old at this point.
Quoted 6 lines
I have a Seagate Barracuda 7200.9 80 Gbyte SATA drive that I use for experiments. I can permanently destroy a EXT3 file-system at least 50% of the time by disconnecting the data cable while a
ddwrite to a file is in progress. Something bad happens making partition information invalid. I have to re-partition to reuse the drive.
Does turning off writeback cache on disk help? This is quite serious, I'd say...
Pavel
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Theodore TsoQuoted 8 lines · Theodore Tso
Quoted 1 line · Bryan Henderson
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Well, it would be impossible or at least very hard to see that in practice, right? My understanding is that drives do sector-level checksums, so if there was a partially written sector, the checksum would be bogus and the drive would return an error when you tried to read from it.
There is extensive per sector error correction on each sector written. What you would see in this case (or many, many other possible ways drives can corrupt media) is a "media error" on the next read.
You would never get back the partially written contents of that sector at the host.
Having our tools (fsck especially) be resilient in the face of media errors is really critical. Although I don't think the scenario of a partially written sector is common, media errors in general are common and can develop over time.
Quoted 9 lines
Quoted 2 lines
Ted brought up the separate issue of the host sending garbage to the disk device because its own power is failing at the same time, which makes the integrity at the disk level moot (or even undesirable, as you'd rather write a bad sector than a good one with the wrong data).
Yep, exactly. It would be interesting to see if this happens on modern hardware; all of the evidence I've had for this is years old at this point.
- Ted
See the NetApp paper from Sigmetrics 2007 for some interesting analysis...
ric
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 11 lines · Ric Wheeler
Quoted 8 lines · Theodore Tso
Quoted 1 line · Bryan Henderson
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Well, it would be impossible or at least very hard to see that in practice, right? My understanding is that drives do sector-level checksums, so if there was a partially written sector, the checksum would be bogus and the drive would return an error when you tried to read from it.
There is extensive per sector error correction on each sector written. What you would see in this case (or many, many other possible ways drives can corrupt media) is a "media error" on the next read.
Correct.
Quoted 1 line
You would never get back the partially written contents of that sector at the host.
Correct.
Quoted 1 line
Having our tools (fsck especially) be resilient in the face of media errors is really critical. Although I don't think the scenario of a partially written sector is common, media errors in general are common and can develop over time.
Agreed.
Jeff
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
I just had a talk with a colleague, John Palmer, who worked on disk drive design for about 5 years in the '90s and he gave me a very confident, credible explanation of some of the things we've been wondering about disk drive power loss in this thread, complete with demonstrations of various generations of disk drives, dismantled.
First of all, it is plain to see that there is no spring capable of parking the head, and there is no capacitor that looks big enough to possibly supply the energy to park the head, in any of the models I looked at. Since parking of the heads is essential, we can only conclude that the myth of the kinetic energy of the disks being used for that (turned into electricity by the drive motor) is true. The energy required is not just to move the heads to the parking zone, but to latch them there as well.
The myth is probably just that that energy is used for anything else; it's really easy to build a dumb circuit to park the heads using that power; keeping a computer running is something else.
The drive does drop a write in the middle of the sector if it is writing at the time of power loss. The designers were too conservative to keep writing as power fails -- there's no telling what damage you might do. So the drive cuts the power to the heads at the first sign of power loss. If a write was in progress, this means there is one garbage sector on the disk. It can't be read.
Trying to finish writing the sector is something I can image some drive model somewhere trying to do, but if even _some_ take the conservative approach, everyone has to design for it, so it doesn't matter.
A device might then reassign that sector the next time you try to write to it (after failing to read it), thinking the medium must be bad. But there are various algorithms for deciding when to reassign a sector, so it might not too.
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Bryan HendersonQuoted 5 lines · Bryan Henderson
We weren't actually talking about writing out the cache. While that was part of an earlier thread which ultimately conceded that disk drives most probably do not use the spinning disk energy to write out the cache, the claim was then made that the drive at least survives long enough to finish writing the sector it was writing, thereby maintaining the integrity of the data at the drive level. People often say that a disk drive guarantees atomic writes at the sector level even in the face of a power failure.
But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector.
Did he work for Maxtor, by any chance? :-/
A disk drive whose power is cut needs to have enough residual power to park its heads (or *massive* data loss will occur), and at that point it might as well keep enough on hand to finish an in-progress sector write.
There are two possible sources of onboard temporary power: a large enough capacitor, or the rotational energy of the platters (an electrical motor also being a generator.) I don't care which one they use, but they need to do something.
-hpa
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
"H. Peter Anvin" <[email protected]> wrote on 01/18/2008 07:08:30 AM:
Quoted 3 lines
Quoted 2 lines · Bryan Henderson
We weren't actually talking about writing out the cache. While that
was
Quoted 1 line
Quoted 1 line
part of an earlier thread which ultimately conceded that disk drives
most
Quoted 1 line
Quoted 1 line
probably do not use the spinning disk energy to write out the cache,
the
Quoted 1 line
Quoted 1 line
claim was then made that the drive at least survives long enough to
finish
Quoted 1 line
Quoted 1 line
writing the sector it was writing, thereby maintaining the integrity
of
Quoted 2 lines
Quoted 2 lines
the data at the drive level. People often say that a disk drive guarantees atomic writes at the sector level even in the face of a
power
Quoted 3 lines
Quoted 3 lines
failure.
But I heard some years ago from a disk drive engineer that that is a
myth
Quoted 1 line
Quoted 1 line
just like the rotational energy thing. I added that to the
discussion,
Quoted 8 lines
Quoted 4 lines
but admitted that I haven't actually seen a disk drive write a partial
sector.
A disk drive whose power is cut needs to have enough residual power to park its heads (or *massive* data loss will occur), and at that point it
Quoted 6 lines
might as well keep enough on hand to finish an in-progress sector write.
There are two possible sources of onboard temporary power: a large enough capacitor, or the rotational energy of the platters (an electrical motor also being a generator.) I don't care which one they use, but they need to do something.
I believe the power for that comes from a third source: a spring. Parking the heads is too important to leave to active circuits.
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Alan CoxQuoted 6 lines · Alan Cox
Quoted 5 lines
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
Quoted 6 lines
AFAIK no drive saves the cache. The worst case cache flush for drives is several seconds with no retries and a couple of minutes if something really bad happens.
This is why the kernel has some knowledge of barriers and uses them to issue flushes when needed.
Problem is, ext3 has barriers off by default so it's not saving most people.
And then if you turn them on, but have your filesystem on an lvm device, lvm strips them out again.
-Eric
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Daniel PhillipsQuoted 9 lines · Daniel Phillips
Quoted 2 lines · Pavel Machek
I had write cache enabled on my main computer. Oops. I guess that means we do need better documentation.
Writeback cache on disk in iteself is not bad, it only gets bad if the disk is not engineered to save all its dirty cache on power loss, using the disk motor as a generator or alternatively a small battery. It would be awfully nice to know which brands fail here, if any, because writeback cache is a big performance booster.
Is it?
I guess I should try to measure it. (Linux already does writeback caching, with 2GB of memory. I wonder how important disks's 2MB of cache can be).
Pavel
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 3 lines · Pavel Machek
I guess I should try to measure it. (Linux already does writeback caching, with 2GB of memory. I wonder how important disks's 2MB of cache can be).
It serves essentially the same purpose as the 'async' option in /etc/exports (i.e. we declare it "done" when the other end of the wire says it's caught the data, not when it's actually committed), with similar latency wins. Of course, it's impedance-matching for bursty traffic - the 2M doesn't do much at all if you're streaming data to it. For what it's worth, the 80G Seagate drive in my laptop claims it has 8M, so it probably does 4 times as much good as 2M. ;)
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Hi!
Quoted 11 lines
Quoted 3 lines
I guess I should try to measure it. (Linux already does writeback caching, with 2GB of memory. I wonder how important disks's 2MB of cache can be).
It serves essentially the same purpose as the 'async' option in
/etc/exports(i.e. we declare it "done" when the other end of the wire says it's caught the data, not when it's actually committed), with similar latency wins. Of course, it's impedance-matching for bursty traffic - the 2M doesn't do much at all if you're streaming data to it. For what it's worth, the 80G Seagate drive in my laptop claims it has 8M, so it probably does 4 times as much good as 2M. ;)
I doubt "impedance-matching" is useful here. SATA link is fast/low latency, and kernel already does buffering with main memory...
Hmm... what is the way to measure that? Tar decompress kernel few times with cache on / cache off?
Pavel
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to David ChinnerQuoted 3 lines · David Chinner
ext3 is not the only filesystem that will have trouble due to volatile write caches. We see problems often enough with XFS due to volatile write caches that it's in our FAQ:
In fact it will hit every filesystem. A write-back cache that can't be forced to write back bythe filesystem will cause corruption on uncontained power loss, period.
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Pavel MachekHi Pavel,
Along with this effort, could you let me know if the world actually cares about online fsck? Now we know how to do it I think, but is it worth the effort.
Regards,
Daniel
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 3 lines · On Tue, 15 Jan 2008 20:44:38 -0500 "Daniel Phillips" <[email protected]>…
Along with this effort, could you let me know if the world actually cares about online fsck? Now we know how to do it I think, but is it worth the effort.
With a filesystem that is compartmentalized and checksums metadata, I believe that an online fsck is absolutely worth having.
Instead of the filesystem resorting to mounting the whole volume read-only on certain errors, part of the filesystem can be offlined while an fsck runs. This could even be done automatically in many situations.
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 7 lines · Rik van Riel
With a filesystem that is compartmentalized and checksums metadata, I believe that an online fsck is absolutely worth having.
Instead of the filesystem resorting to mounting the whole volume read-only on certain errors, part of the filesystem can be offlined while an fsck runs. This could even be done automatically in many situations.
In ext4 we store per-group state flags in each group, and the group descriptor is checksummed (to detect spurious flags), so it should be relatively straight forward to store an "error" flag in a single group and have it become read-only.
As a starting point, it would be worthwhile to check instances of ext4_error() to see how many of them can be targetted at a specific group. I'd guess most of them could be (corrupt inodes, directory and indirect blocks, incorrect bitmaps).
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Daniel PhillipsHi!
Quoted 2 lines
Along with this effort, could you let me know if the world actually cares about online fsck?
I'm not the world's spokeperson (yet ;-).
Quoted 2 lines
Now we know how to do it I think, but is it
worth the effort.
ext3's "lets fsck on every 20 mounts" is good idea, but it can be annoying when developing. Having option to fsck while filesystem is online takes that annoyance away.
So yes, it would be very useful for me...
For long-running servers, this may be less of a problem... but OTOH their filesystems are not checked at all as long servers are online... so online fsck is actually important there, too, but for other reasons.
So yes, it is very useful for world.
Pavel
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Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 4 lines · Pavel Machek
ext3's "lets fsck on every 20 mounts" is good idea, but it can be annoying when developing. Having option to fsck while filesystem is online takes that annoyance away.
I'm sure everyone on cc: knows this, but for the record you can change ext3's fsck on N mounts or every N days to something that makes sense for your use case. Usually I just turn it off entirely and run fsck by hand when I'm worried:
# tune2fs -c 0 -i 0 /dev/whatever
-VAL
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
In reply to Daniel PhillipsQuoted 3 lines · Daniel Phillips
Along with this effort, could you let me know if the world actually cares about online fsck? Now we know how to do it I think, but is it worth the effort.
Most users seem to care deeply about "things just work". Here is why ntfs-3g also took the online fsck path some time ago.
NTFS support had a highly bad reputation on Linux thus the new code was written with rigid sanity checks and extensive automatic, regression testing. One of the consequences is that we're detecting way too many inconsistencies left behind by the Windows and other NTFS drivers, hardware faults, device drivers.
To better utilize the non-existing developer resources, it was obvious to suggest the already existing Windows fsck (chkdsk) in such cases. Simple and safe as most people like us would think who never used Windows.
However years of experience shows that depending on several factors chkdsk may start or not, may report the real problems or not, but on the other hand it may report bogus issues, it may run long or just forever, and it may even remove completely valid files. So one could perhaps even consider suggestions to run chkdsk a call to play Russian roulette.
Thankfully NTFS has some level of metadata redundancy with signatures and weak "checksums" which make possible to correct some common and obvious corruptions on the fly.
Similarly to ZFS, Windows Server 2008 also has self-healing NTFS:
http://technet2.microsoft.com/windowsserver2008/en/library/6f883d0d-3668-4e15-b7ad-4df0f6e6805d1033.mspx?mfr=true
Szaka
--
NTFS-3G: http://ntfs-3g.org
Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Quoted 1 line · Szabolcs Szakacsits
Similarly to ZFS, Windows Server 2008 also has self-healing NTFS:
I guess that is enough votes to justify going ahead and trying an implementation of the reverse mapping ideas I posted. But of course more votes for this is better. If online incremental fsck is something people want, then please speak up here and that will very definitely help make it happen.
On the walk-before-run principle, it would initially just be filesystem checking, not repair. But even this would help, by setting per-group checked flags that offline fsck could use to do a much quicker repair pass. And it will let you know when a volume needs to be taken offline without having to build in planned downtime just in case, which already eats a bunch of nines.
Regards,
Daniel
Quoted 17 lines · Pavel Machek
Quoted 8 lines · Theodore Tso
Quoted 4 lines · Al Boldi
Ok, but let's look at this a bit more opportunistic / optimistic.
Even after a black-out shutdown, the corruption is pretty minimal, using ext3fs at least.
After a unclean shutdown, assuming you have decent hardware that doesn't lie about when blocks hit iron oxide, you shouldn't have any corruption at all. If you have crappy hardware, then all bets are off....
What hardware is crappy here. Lets say... internal hdd in thinkpad x60?
What are ext3 expectations of disk (is there doc somewhere)? For example... if disk does not lie, but powerfail during write damages the sector -- is ext3 still going to work properly?
Quoted 8 lines
If disk does not lie, but powerfail during write may cause random numbers to be returned on read -- can fsck handle that?
What abou disk that kills 5 sectors around sector being written during powerfail; can ext3 survive that?
Pavel
I think that you have to keep in mind the way disk (and other media) fail. You can get media failures after a successful write or errors that pop up as the media ages.
Not to mention the way most people run with write cache enabled and no write barriers enabled - a sure recipe for corruption.
Of course, there are always software errors to introduce corruption even when we get everything else right ;-)
From what I see, media errors are the number one cause of corruption in file systems. It is critical that fsck (and any other tools) continue after an IO error since they are fairly common (just assume that sector is lost and do your best as you continue on).
ric
Hi Ted,
Quoted 8 lines · Theodore Tso
What is very hard to check is whether or not the link count on the inode is correct. Suppose the link count is 1, but there are actually two directory entries pointing at it. Now when someone unlinks the file through one of the directory hard entries, the link count will go to zero, and the blocks will start to get reused, even though the inode is still accessible via another pathname. Oops. Data Loss.
Quoted 9 lines
This is why doing incremental, on-line fsck'ing is *hard*. You're not going to find this while doing each directory one at a time, and if the filesystem is changing out from under you, it gets worse. And it's not just the hard link count. There is a similar issue with the block allocation bitmap. Detecting the case where two files are simultaneously can't be done if you are doing it incrementally, and if the filesystem is changing out from under you, it's impossible, unless you also have the filesystem telling you every single change while it is happening, and you keep an insane amount of bookkeeping.
In this case I am listening to Chicken Little carefully and really do believe the sky will fall if we fail to come up with an incremental online fsck some time in the next few years. I realize the challenge verges on insane, but I have been slowly chewing away at this question for some time.
Val proposes to simplify the problem by restricting the scope of block pointers and hard links. Best of luck with that, the concept of fault isolation domains has a nice ring to it. I prefer to stick close to tried and true Ext3 and not change the basic algorithms.
Rather than restricting pointers, I propose to add a small amount of new metadata to accelerate global checking. The idea is to be able to build per-group reverse maps very quickly, to support mapping physical blocks back to inodes that own them, and mapping inodes back to the directories that reference them.
I see on-the-fly filesystem reverse mapping as useful for more than just online fsck. For example it would be nice to be able to work backwards efficiently from a list of changed blocks such as ddsnap produces to a list of file level changes.
The amount of metadata required to support efficient on-the-fly reverse mapping is surprisingly small: 2K per block group per terabyte, in a fixed location at the base of each group. This is consistent with my goal of producing code that is mergable for Ext4 and backportable to Ext3.
Building a block reverse map for a given group is easy and efficient. The first pass walks across the inode table and already maps most of the physical blocks for typical usage patterns, because most files only have direct pointers. Index blocks discovered in the first pass go onto a list to be processed by subsequent passes, which may discover additional index blocks. Just keep pushing the index blocks back onto the list and the algorithm terminates when the list is empty. This builds a reverse map for the group including references to external groups.
Note that the recent metadata clustering patch from Abhishek Rai will speed up this group mapping algorithm significantly because (almost)
all the index blocks can be picked up in one linear read. This should
only take a few milliseconds. One more reason why I think his patch is an Important Patch[tm].
A data block may be up to four groups removed from its home group, therefore the reverse mapping process must follow pointers across groups and map each file entirely to be sure that all pointers to the group being checked have been discovered. It is possible to construct a case where a group contains a lot of inodes of big files that are mostly stored in other groups. Mapping such a group could possibly require examining all the index blocks on the entire volume. That would be about 2**18 index blocks per terabyte, which is still within the realm of practicality.
To generate the inode reverse map for, walk each directory in the group, decoding the index blocks by hand. Strictly speaking, directories ought to pass block level checking before being reverse mapped, but there could be many directories in the same group spilling over into a lot of external groups, so getting all the directory inodes to pass block level checks at the same time could be difficult with filesystem writing going on between fsck episodes. Instead, just go ahead and assume a directory file is ok, and if this is not the case the directory walk will fail or a block level check will eventually pick up the problem.
The worst case for directory mapping is much worse than the worst case for block mapping. A single directory could fill an entire volume. For such a large directory, reverse mapping is not possible without keeping the filesystem suspended for an unreasonable time. Either make the reverse map incremental and maintained on the fly or fall back to a linear search of the entire directory when doing the checks below, the latter being easy but very slow. Or just give up on fscking groups involving the directory. Or maybe I am obsessing about this too much, because mapping a directory of a million files only requires reading about 60 MB, and such large directories are very rare.
The group cross reference tables have to be persistently recorded on disk in order to avoid searching the whole volume for some checks. A per group bitmap handles this nicely, with as many bits as there are block groups. Each one bit flags some external group as referencing the group in which the bitmap is stored. With default settings, a cross reference bitmap is only 1K per terabyte. Two such bitmaps are needed per group, one for external block pointers and the other for external hard links. When needed for processing, a bitmap is converted into a list or hash table. New cross group references need to be detected in the filesystem code and saved to disk before the associated transaction proceeds. Though new cross group references should be relatively rare, the cross reference bitmaps can be logically journalled in the commit block of the associated transaction and batch-updated on journal flush so that there is very little new write overhead.
Cross group references may be updated lazily on delete, since the only harm caused by false positives is extra time spent building unneeded reverse maps. The incremental fsck step "check group cross reference bitmaps" describes how redundant cross reference bits are detected. These can be batched up in memory and updated as convenient.
Cached reverse maps can be disturbed by write activity on the filesystem. The lazy approach is to discard them on any change, which should work well enough to get started. With additional work, cached reverse maps can be updated on the fly by the respective get_block, truncate and directory entry operations.
The incremental fsck algorithm works by checking a volume one block group at a time, with filesystem operations suspended during the check. The expected service interruption will be small compared to taking the volume offline but will occur more often, which might be an issue for interactive use. Making the algorithm completely bumpless would require something like a temporary in-memory volume snapshot, followed by a clever merge of the changed blocks, taking advantage of the reverse maps to know which checked groups need to be changed back to unchecked. Beyond the scope of the current effort.
Broadly, there are two layers of integrity to worry about:
1) Block pointers (block level)
2) Directory entries (inode level)
Luckily for lazy programmers, similar techniques and even identical data structures work for both. Each group has one persistent bitmap to reference the group via block pointers, and another to show which groups reference the group via directory entries. In memory, there is one cached reverse map per group to map blocks to inodes (one to one), and another to map inodes to directory inodes (one to many). Algorithms for block and inode level checks are similar, as detailed below.
With on-demand reverse maps to help us, we do something like:
* Suspend filesystem, flushing dirty page cache to disk
* Build reverse map for this group if needed
* Build reverse maps for groups referencing this group as needed
* Perform checks listed below
* If the checks passed mark the group as checked
* Resume filesystem
The order in which groups are checked can depend opportunistically on which reverse maps are already cached. Some kind of userspace interface would let the operator know about checking progress and the nature of problems detected. Each group records the time last checked, the time checked successfully, and a few bits indicating the nature of problems found.
Now for the specific integrity checks, and strategy for each. There are two interesting kinds of potentially nonlocal checks:
Downward check: this group may reference other groups that must be
examined together with this group to complete the check. Can be completed immediately when all references are local, otherwise make a list of groups needed to continue the check and defer until convenient to map those groups all at the same time.
Upward check: this group may be referenced by other groups that must
be examined together with this group in order to complete the check. Need to check the maps of all groups referencing this group to find incoming references.
To prepare for checking a group, its reverse map is constructed if not already cached, and the reverse maps for all groups marked as referencing the group. If that is too many reverse maps then just give up on trying to fsck that group, or do it very slowly by constructing each reverse map at the point it is actually needed in a check.
As online fsck works its way through groups a list of pending downward checks for certain inodes will build up. When this list gets long enough, find a subset of it involving a reasonably small number of groups, map those groups and perform the needed checks.
A list of checks and correspondence to e2fsck passes follows.
Inode mode field size and block count (e2fsck pass 1)
Downward check. Do the local inode checks, then walk the inode index structure counting the blocks. Ensure that the rightmost block lies within the inode size.
Check block references (e2fsck pass 1)
Upward check. Check that each block found to be locally referenced is not marked free in the block bitmap. For each block found to have no local reference, check the maps of the groups referencing this group to ensure that exactly one of them points at the block, or none if the block is marked free in the group bitmap.
Check directory structure (e2fsck pass 2)
Downward check. The same set of directory structure tests as e2fsck, such as properly formed directory entries, htree nodes, etc.
Check directory inode links (e2fsck pass 3)
Upward check. While walking directory entries, ensure that each directory inode to be added to the reverse map is not already in the map and is not marked free in the inode bitmap. For each inode discovered to have no local link after building the reverse map, check the reverse maps of the groups referring to this group to ensure that exactly one of them links to the inode, or that there are no external links if the block bitmap indicates the block is free.
Check inode reference counts (e2fsck pass 4)
Upward check. While walking directory entries, ensure that each non directory inode to be added to the reverse map is not marked free in the inode bitmap. Check that the inode reference count is equal to the number of references to the inode found in the local reverse map plus the number of references found in the maps of all groups referencing this group.
Check block bitmaps (e2fsck pass 5)
Checking block references above includes ensuring that no block in use
is marked free. Now check that no block marked free in the block
bitmap appears in the local or external block reverse map.
Check inode bitmaps (e2fsck pass 5)
Checking inode references above includes ensuring that no inode in use
is marked free. Now check that no inode marked free in the inode
bitmap appears in the local or external inode reverse map.
Check group cross reference bitmaps
Each time a group is mapped, check that for each external reference discovered the corresponding bit is set in the external bitmap. Check that for all groups having an external reference bit set for this group, this group does in fact reference the external group. Because cross reference bitmaps are so small they should all fit in cache comfortably. The buffer cache is ideal for this.
Finally...
Total work required to get something working along these lines looks significant, but the importance is high, so work is quite likely to go ahead if the approach survives scrutiny.
Regards,
Daniel
Quoted 2 lines · Al Boldi
But why wouldn't it be possible to do this on the current fs infrastructure, using just a smart fsck, working incrementally on some sub-dir?
If you have /home/usera, /home/userb, and /home/userc, the vast majority of fs screw-ups can't be detected by only looking at one sub-dir. For example, you can't tell definitively that all blocks referenced by an inode under /home/usera are properly only allocated to one file until you *also* look at the inodes under user[bc]. Heck, you can't even tell if the link count for a file is correct unless you walk the entire filesystem - you can find a file with a link count of 3 in the inode, and you find one reference under usera, and a second under userb - you can't tell if the count is one too high or not until you walk through userc and actually see (or fail to see) a third directory entry referencing it.
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Quoted 10 lines · Andi Kleen
Quoted 3 lines · Theodore Tso
Now, there are good reasons for doing periodic checks every N mounts and after M months. And it has to do with PC class hardware. (Ted's
aphorism: "PC class hardware is cr*p").If these reasons are good ones (some skepticism here) then the correct way to really handle this would be to do regular background scrubbing during runtime; ideally with metadata checksums so that you can actually detect all corruption.
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But since fsck is so slow and disks are so big this whole thing is a ticking time bomb now. e.g. it is not uncommon to require tens of minutes or even hours of fsck time and some server that reboots only every few months will eat that when it happens to reboot. This means you get a quite long downtime.
Has there been some thought about an incremental fsck?
Is that anything like a cluster fsck? ]:>
Quoted 10 lines · Andi Kleen
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Quoted 3 lines · Theodore Tso
Now, there are good reasons for doing periodic checks every N mounts and after M months. And it has to do with PC class hardware. (Ted's
aphorism: "PC class hardware is cr*p").If these reasons are good ones (some skepticism here) then the correct way to really handle this would be to do regular background scrubbing during runtime; ideally with metadata checksums so that you can actually detect all corruption.
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But since fsck is so slow and disks are so big this whole thing is a ticking time bomb now. e.g. it is not uncommon to require tens of minutes or even hours of fsck time and some server that reboots only every few months will eat that when it happens to reboot. This means you get a quite long downtime.
Has there been some thought about an incremental fsck?
While an _incremental_ fsck isn't so easy for existing filesystem types, what is pretty easy to automate is making a read-only snapshot of a filesystem via LVM/DM and then running e2fsck against that. The kernel and filesystem have hooks to flush the changes from cache and make the on-disk state consistent.
You can then set the the ext[234] superblock mount count and last check time via tune2fs if all is well, or schedule an outage if there are inconsistencies found.
There is a copy of this script at:
http://osdir.com/ml/linux.lvm.devel/2003-04/msg00001.html
Note that it might need some tweaks to run with DM/LVM2 commands/output, but is mostly what is needed.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
Quoted 10 lines · Andreas Dilger
While an _incremental_ fsck isn't so easy for existing filesystem types, what is pretty easy to automate is making a read-only snapshot of a filesystem via LVM/DM and then running e2fsck against that. The kernel and filesystem have hooks to flush the changes from cache and make the on-disk state consistent.
You can then set the the ext[234] superblock mount count and last check time via tune2fs if all is well, or schedule an outage if there are inconsistencies found.
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There is a copy of this script at:
http://osdir.com/ml/linux.lvm.devel/2003-04/msg00001.htmlNote that it might need some tweaks to run with DM/LVM2 commands/output, but is mostly what is needed.
You can do this now with ddsnap (an out-of-tree device mapper target) either by checking a local snapshot or a replicated snapshot on a different machine, see:
Doing the check on a remote machine seems attractive because the fsck does not create a load on the server.
Regards,
Daniel