XFS speculative preallocation
Hi all,
Eric had suggested we add an FAQ entry for speculative preallocation since it seems to be a common question, so I offered to write something up. I started with a single entry but split it into a couple Q's when it turned into TL;DR fodder. ;)
The text is embedded below for review. Thoughts on the questions or content is appreciated. Also, once folks are Ok with this... how does one gain edit access to the wiki?
Brian
---
Q: Why do files on XFS use more data blocks than expected?
A:
The XFS speculative preallocation algorithm allocates extra blocks beyond end of file (EOF) to combat fragmentation under parallel sequential write workloads. This post-EOF block allocation is included in 'st_blocks' counts via stat() system calls and is accounted as globally allocated space by the filesystem. This is reported by various userspace utilities (stat, du, df, ls) and thus provides a common source of confusion for administrators. Post-EOF blocks are temporary in most situations and are usually reclaimed via several possible mechanisms in XFS.
See the FAQ entry on speculative preallocation for details.
Q: What is speculative preallocation? How can I manage it?
A:
XFS speculatively preallocates post-EOF blocks on file extending writes in anticipation of future extending writes. The size of a preallocation is dynamic and depends on the size of the previous extent in the file (starting from 0 again if the write extends past a hole). As files grow larger, so do the size of preallocations. Speculative preallocation is not enabled for files smaller than a minimum size (64k by default, but can vary depending on filesystem geometry and/or mount options). Preallocations are capped at a maximum of 8GB on 4k block filesystems. Preallocation is throttled automatically as the filesystem approaches low free space conditions or other allocation limits on a file (such as a quota).
In most cases, speculative preallocation is automatically reclaimed when a file is closed. The preallocation may persist after file close if an open, write, close pattern is repeated on a file. In this scenario, post-EOF preallocation is trimmed once the inode is reclaimed from cache or the filesystem unmounted.
Linux 3.8 (and later) includes a scanner to perform background trimming of files with lingering post-EOF preallocations. The scanner bypasses files that have been recently modified to not interfere with ongoing writes. A 5 minute scan interval is used by default and can be adjusted via the following file (value in seconds):
/proc/sys/fs/xfs/speculative_prealloc_lifetime
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface. Preallocated space can also be encoded permanently in situations where file size is extended beyond a range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated blocks are reclaimed on file close, inode reclaim, unmount or in the background once file write activity subsides.
Finally, the XFS block allocation algorithm can be configured to use a fixed allocation size with the 'allocsize=' mount option. Note that speculative preallocation does not occur when a fixed allocation size is set and thus increases the potential for fragmentation via parallel writes.
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Brian,
FWIW, from my perspective as a newcomer to XFS that is quite clear and understandable and informative. Looks like a valuable addition.
I've got no idea how to get write access on the wiki personally, but hopefully that answer will arrive for you 'soon(tm)'.
Cheers,
-Shaun
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Brian Foster
Sent: Friday, March 21, 2014 11:29 AM
To: [email protected]
Subject: [FAQ] XFS speculative preallocation
Hi all,
Eric had suggested we add an FAQ entry for speculative preallocation since it seems to be a common question, so I offered to write something up. I started with a single entry but split it into a couple Q's when it turned into TL;DR fodder. ;)
The text is embedded below for review. Thoughts on the questions or content is appreciated. Also, once folks are Ok with this... how does one gain edit access to the wiki?
Brian
---
Q: Why do files on XFS use more data blocks than expected?
A:
The XFS speculative preallocation algorithm allocates extra blocks beyond end of file (EOF) to combat fragmentation under parallel sequential write workloads. This post-EOF block allocation is included in 'st_blocks' counts via stat() system calls and is accounted as globally allocated space by the filesystem. This is reported by various userspace utilities (stat, du, df, ls) and thus provides a common source of confusion for administrators. Post-EOF blocks are temporary in most situations and are usually reclaimed via several possible mechanisms in XFS.
See the FAQ entry on speculative preallocation for details.
Q: What is speculative preallocation? How can I manage it?
A:
XFS speculatively preallocates post-EOF blocks on file extending writes in anticipation of future extending writes. The size of a preallocation is dynamic and depends on the size of the previous extent in the file (starting from 0 again if the write extends past a hole). As files grow larger, so do the size of preallocations. Speculative preallocation is not enabled for files smaller than a minimum size (64k by default, but can vary depending on filesystem geometry and/or mount options). Preallocations are capped at a maximum of 8GB on 4k block filesystems.
Preallocation is throttled automatically as the filesystem approaches low free space conditions or other allocation limits on a file (such as a quota).
In most cases, speculative preallocation is automatically reclaimed when a file is closed. The preallocation may persist after file close if an open, write, close pattern is repeated on a file. In this scenario, post-EOF preallocation is trimmed once the inode is reclaimed from cache or the filesystem unmounted.
Linux 3.8 (and later) includes a scanner to perform background trimming of files with lingering post-EOF preallocations. The scanner bypasses files that have been recently modified to not interfere with ongoing writes. A 5 minute scan interval is used by default and can be adjusted via the following file (value in seconds):
/proc/sys/fs/xfs/speculative_prealloc_lifetime
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface. Preallocated space can also be encoded permanently in situations where file size is extended beyond a range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated blocks are reclaimed on file close, inode reclaim, unmount or in the background once file write activity subsides.
Finally, the XFS block allocation algorithm can be configured to use a fixed allocation size with the 'allocsize=' mount option. Note that speculative preallocation does not occur when a fixed allocation size is set and thus increases the potential for fragmentation via parallel writes.
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_______________________________________________
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[email protected]
http://oss.sgi.com/mailman/listinfo/xfs
Quoted 10 lines · Brian Foster
Hi all,
Eric had suggested we add an FAQ entry for speculative preallocation since it seems to be a common question, so I offered to write something up. I started with a single entry but split it into a couple Q's when it turned into TL;DR fodder. ;)
The text is embedded below for review. Thoughts on the questions or content is appreciated. Also, once folks are Ok with this... how does one gain edit access to the wiki?
More questions or topics that can be converted to questions from me:
1) Before preallocation kernel did things differently. AFAIK it wasn't the same as allocsize=64k, was it? Is there a way to get old behaviour or something similar to old behaviour?
2) Is there a way to see which file got some preallocation and how big that preallocation is? Scenario - something ate free space due to preallocation and from admin point of view it would be usefull to know which app did that and how many MB was due to preallocation (vs real, written data).
Quoted 3 lines
Linux 3.8 (and later) includes a scanner to perform background trimming of files with lingering post-EOF preallocations. The scanner bypasses files that have been recently
What time is "recently" ? Is "modified" equal to "file data modified" or "file data or metadata modified" ?
Quoted 2 lines
modified to not interfere with ongoing
writes.
In case of some app that constantly writes to files (apache web server writting to its logs for example) that background trimming will never do anything for these files, right?
Quoted 5 lines
A 5 minute scan interval is used by default and can be adjusted via the following file (value in seconds):
/proc/sys/fs/xfs/speculative_prealloc_lifetime
Quoted 7 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface. Preallocated space can also be encoded permanently in situations where file size is extended beyond a range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated blocks are reclaimed on file close, inode reclaim, unmount or in the background once file write activity subsides.
So there is no mechanism that would shirnk preallocations in case when free space is (almost or) gone on a fs? Case: apache causes xfs to preallocate several GB for its /var/..../{access,error}_log (common problem here) and then free space ends on that fs causing problems for every app that writes to /var.
Thanks!
Signature · 6 lines
Arkadiusz Miśkiewicz, arekm / maven.pl _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
Quoted 12 lines · Arkadiusz Miśkiewicz
Quoted 10 lines · Brian Foster
Hi all,
Eric had suggested we add an FAQ entry for speculative preallocation since it seems to be a common question, so I offered to write something up. I started with a single entry but split it into a couple Q's when it turned into TL;DR fodder. ;)
The text is embedded below for review. Thoughts on the questions or content is appreciated. Also, once folks are Ok with this... how does one gain edit access to the wiki?
Quoted 6 lines
More questions or topics that can be converted to questions from me:
1) Before preallocation kernel did things differently. AFAIK it wasn't the same as allocsize=64k, was it? Is there a way to get old behaviour or something similar to old behaviour?
Going from the commit log that introduced speculative preallocation, it appears that the behavior was effectively allocsize=64k. For reference:
055388a3 xfs: dynamic speculative EOF preallocation
Quoted 5 lines
2) Is there a way to see which file got some preallocation and how big that preallocation is? Scenario - something ate free space due to preallocation and from admin point of view it would be usefull to know which app did that and how many MB was due to preallocation (vs real, written data).
The common scenario is when du/stat reports a larger block usage than file size, so the question of how much extra space is allocated is just the difference between the two. I suppose we could include a simple example of that in the first Q.
This isn't necessarily true in the case of sparse files. xfs_bmap prints the extent information for a file, so it should be possible to determine how much post-EOF space exists from looking at the extent that covers EOF. That said, this strikes me as more "user guide" material than FAQ.
Quoted 7 lines
Quoted 3 lines
Linux 3.8 (and later) includes a scanner to perform background trimming of files with lingering post-EOF preallocations. The scanner bypasses files that have been recently
What time is "recently" ? Is "modified" equal to "file data modified" or "file data or metadata modified" ?
I originally had something like "files that have not been modified since last flushed to disk," which is the heuristic as I understand it. That seemed too verbose and technical for FAQ. I could replace "recently modified" with "... bypasses files that are dirty ..." if that is more useful..?
Quoted 7 lines
Quoted 2 lines
modified to not interfere with ongoing
writes.In case of some app that constantly writes to files (apache web server writting to its logs for example) that background trimming will never do anything for these files, right?
Most likely true. Though by the same logic, those files will eventually use the preallocated space.
Quoted 13 lines
Quoted 5 lines
A 5 minute scan interval is used by default and can be adjusted via the following file (value in seconds):
/proc/sys/fs/xfs/speculative_prealloc_lifetimeQuoted 7 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface. Preallocated space can also be encoded permanently in situations where file size is extended beyond a range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated blocks are reclaimed on file close, inode reclaim, unmount or in the background once file write activity subsides.
Quoted 5 lines
So there is no mechanism that would shirnk preallocations in case when free space is (almost or) gone on a fs? Case: apache causes xfs to preallocate several GB for its
/var/..../{access,error}_log(common problem here) and then free space ends on that fs causing problems for every app that writes to /var.
I noted in the second answer that the preallocation is throttled as we near allocation limits such as no free space or quota. I think that should cover most cases. I still have some code lying around somewhere that forces a scan and retry in EDQUOT scenarios though. I should dust that off...
Thanks for the reviews!
Brian
Quoted 9 lines
Thanks!
Signature · 6 lines
Arkadiusz Miśkiewicz, arekm / maven.pl _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
_______________________________________________
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[email protected]
http://oss.sgi.com/mailman/listinfo/xfs
* Brian Foster:
Quoted 3 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface.
How does an explicit allocation with posix_fallocate interact with speculative preallocation? Does it disable it?
I see rather dramatic fragmentation of the systemd journal when it is stored on XFS, and it calls posix_fallocate before writing data to the file.
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Quoted 10 lines · Brian Foster
Hi all,
Eric had suggested we add an FAQ entry for speculative preallocation since it seems to be a common question, so I offered to write something up. I started with a single entry but split it into a couple Q's when it turned into TL;DR fodder. ;)
The text is embedded below for review. Thoughts on the questions or content is appreciated. Also, once folks are Ok with this... how does one gain edit access to the wiki?
Request an account and wait for one of us admins to ack it.
FWIW, what I'd really like is for the FAQ to be converted to a asciidoc document in the xfs-documentation tree. The current FAQ has lots of stuff that could do with updating, but editing a wiki document that long in a browser is, well, painful. We can then publish the build html version of the FAQ on the wiki...
Quoted 11 lines
Brian
---
Q: Why do files on XFS use more data blocks than expected?
A:
The XFS speculative preallocation algorithm allocates extra blocks beyond end of file (EOF) to combat fragmentation under parallel sequential write workloads.
"minimise file fragmentation during buffered write workloads. Workloads that benefit from this behaviour include slowly growing files, concurrent writers and mixed reader/writers workloads. It also provides fragmentation resistence in situations where memory pressure prevents adequate buffering of dirty data to allow large contiguous regions of dirty data to be formed in memory."
Quoted 1 line
This post-EOF block allocation is included
"is accounted identically to blocks withing EOF. It is visible..."
Quoted 6 lines
in '
st_blocks' counts viastat()system calls and is accounted as globally allocated space by the filesystem. This is reported by various userspace utilities (stat, du, df, ls) and thus provides a common source of confusion for administrators. Post-EOF blocks are temporary in most situations and are usually reclaimed via several possible mechanisms in XFS.
Also accounted for in quotas.
Quoted 10 lines
See the FAQ entry on speculative preallocation for details.
Q: What is speculative preallocation? How can I manage it?
A:
XFS speculatively preallocates post-EOF blocks on file extending writes in anticipation of future extending writes. The size of a preallocation is dynamic and depends on the size of the previous extent in the file (starting from 0 again if the write extends past a hole).
I'd keep specific heuristics out of the description. Heuristics change....
Quoted 4 lines
As files grow
larger, so do the size of preallocations. Speculative preallocation is not enabled for files smaller than a minimum size (64k by default, but can vary depending on filesystem geometry and/or mount options).
Again, actual numbers should probably be avoided, because we can change that at will...
Quoted 1 line
Preallocations are capped at a maximum of 8GB on 4k block filesystems.
"capped at a single extent of the maximum supported size of the filesystem"
Quoted 3 lines
Preallocation is throttled automatically as the filesystem approaches low free space conditions or other allocation limits on a file (such as a quota).
"Preallocation size is throttled..."
Quoted 5 lines
In most cases, speculative preallocation is automatically reclaimed when a file is closed. The preallocation may persist after file close if an open, write, close pattern is repeated on a file. In this scenario, post-EOF preallocation is trimmed once the inode is reclaimed from cache or the filesystem unmounted.
I'd rewrite this slightly differently, saying that preallocation "may persist beyond the lifecycle of any given file descriptor." And then describe the reason for this - that certain application behaviours (like slowly growing files, or file servers) can cause fragmentation if we remove the preallocation on fd close. These behaviours are automatically detected, and result in "delayed removal" of the preallocation.
Q: How can I speed up or avoid delayed removal of speculative preallocation?
A. Removing the inode from the VFS cache or unmounting the filesystem will remove speculative preallocations associated with an inode.
Quoted 8 lines
Linux 3.8 (and later) includes a scanner to perform background trimming of files with lingering post-EOF preallocations. The scanner bypasses files that have been recently modified to not interfere with ongoing writes. A 5 minute scan interval is used by default and can be adjusted via the following file (value in seconds):
/proc/sys/fs/xfs/speculative_prealloc_lifetime
Q: Is speculative preallocation permanent?
A:
Quoted 8 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface. Preallocated space can also be encoded permanently in situations where file size is extended beyond a range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated blocks are reclaimed on file close, inode reclaim, unmount or in the background once file write activity subsides.
Q: My workload has known characteristics - can I tune speculative preallocation to be an optimal fixed size?
A.
Quoted 5 lines
Finally, the XFS block allocation algorithm can be configured to use a fixed allocation size with the 'allocsize=' mount option. Note that speculative preallocation does not occur when a fixed allocation size is set and thus increases the potential for fragmentation via parallel writes.
This should say "dynamic resizing of speculative preallocation does not occur" rather than "speculative preallocation does not occur", because allocsize only determines the size of the speculative preallocation beyond EOF that is done - it doesn't turn it off...
Cheers,
Dave.
Signature · 7 lines
Dave Chinner [email protected] _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
Quoted 8 lines · Florian Weimer
* Brian Foster:
Quoted 3 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface.
How does an explicit allocation with
posix_fallocateinteract with speculative preallocation? Does it disable it?
fallocate is permanent preallocation using unwritten extents. Speculative preallocation is an extension of delayed allocation that is done when extending the file and the EOF falls into a hole. If there is unwritten extents beyond EOF, speulative preallocation is not performed.
Quoted 3 lines
I see rather dramatic fragmentation of the systemd journal when it is stored on XFS, and it calls
posix_fallocatebefore writing data to the file.
There's your problem - systemd is preventing delayed allocation, and so it fragmenting the file itself with it's write pattern. Basically, that's a bug in systemd, and not something the filesystem can avoid because userspace is directly controlling block allocation.
Cheers,
Dave.
Signature · 7 lines
Dave Chinner [email protected] _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
Quoted 20 lines · Dave Chinner
Quoted 8 lines · Florian Weimer
* Brian Foster:
Quoted 3 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface.
How does an explicit allocation with
posix_fallocateinteract with speculative preallocation? Does it disable it?fallocate is permanent preallocation using unwritten extents. Speculative preallocation is an extension of delayed allocation that is done when extending the file and the EOF falls into a hole. If there is unwritten extents beyond EOF, speulative preallocation is not performed.
Quoted 3 lines
I see rather dramatic fragmentation of the systemd journal when it is stored on XFS, and it calls
posix_fallocatebefore writing data to the file.
Quoted 5 lines
There's your problem - systemd is preventing delayed allocation, and so it fragmenting the file itself with it's write pattern. Basically, that's a bug in systemd, and not something the filesystem can avoid because userspace is directly controlling block allocation.
hohum, I guess we should look into this.
OTOH: nothing wrong with calling posix_fallocate() if you need the space guarantees it provides for proper operation...
-Eric
Quoted 4 lines
Cheers,
Dave.
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Quoted 12 lines · Arkadiusz Miśkiewicz
Quoted 10 lines · Brian Foster
Hi all,
Eric had suggested we add an FAQ entry for speculative preallocation since it seems to be a common question, so I offered to write something up. I started with a single entry but split it into a couple Q's when it turned into TL;DR fodder. ;)
The text is embedded below for review. Thoughts on the questions or content is appreciated. Also, once folks are Ok with this... how does one gain edit access to the wiki?
Quoted 5 lines
More questions or topics that can be converted to questions from me:
1) Before preallocation kernel did things differently. AFAIK it wasn't the same as allocsize=64k, was it? Is there a way to get old behaviour or something similar to old behaviour?
The old behaviour is exactly that of allocsize=64k.
Quoted 6 lines
Quoted 2 lines
modified to not interfere with ongoing
writes.In case of some app that constantly writes to files (apache web server writting to its logs for example) that background trimming will never do anything for these files, right?
If the inode is being constantly dirtied, then the speculative prealloc will not be removed by the background scanner. It only removes prealloc from clean inodes.
Quoted 13 lines
Quoted 5 lines
A 5 minute scan interval is used by default and can be adjusted via the following file (value in seconds):
/proc/sys/fs/xfs/speculative_prealloc_lifetimeQuoted 7 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface. Preallocated space can also be encoded permanently in situations where file size is extended beyond a range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated blocks are reclaimed on file close, inode reclaim, unmount or in the background once file write activity subsides.
Quoted 2 lines
So there is no mechanism that would shirnk preallocations in case when free space is (almost or) gone on a fs?
Background space trimmer takes care of that. We could probably also trigger it on ENOSPC, but once you are already at ENOSPC it's too late....
Quoted 3 lines
Case: apache causes xfs to preallocate
several GB for its/var/..../{access,error}_log(common problem here) and then free space ends on that fs causing problems for every app that writes to /var.
Your log files would have to already be GB in size for that your apache logs to preallocate that much. If your log files are that big, then /var needs to be much, much larger than what the speculative prealloc for a handful of files could easily exhaust.
Cheers,
Dave.
Signature · 7 lines
Dave Chinner [email protected] _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
Quoted 27 lines · Eric Sandeen
Quoted 20 lines · Dave Chinner
Quoted 8 lines · Florian Weimer
* Brian Foster:
Quoted 3 lines
Although speculative preallocation can lead to reports of excess space usage, the preallocated space is not permanent unless explicitly made so via fallocate or a similar interface.
How does an explicit allocation with
posix_fallocateinteract with speculative preallocation? Does it disable it?fallocate is permanent preallocation using unwritten extents. Speculative preallocation is an extension of delayed allocation that is done when extending the file and the EOF falls into a hole. If there is unwritten extents beyond EOF, speulative preallocation is not performed.
Quoted 3 lines
I see rather dramatic fragmentation of the systemd journal when it is stored on XFS, and it calls
posix_fallocatebefore writing data to the file.Quoted 5 lines
There's your problem - systemd is preventing delayed allocation, and so it fragmenting the file itself with it's write pattern. Basically, that's a bug in systemd, and not something the filesystem can avoid because userspace is directly controlling block allocation.
Quoted 4 lines
hohum, I guess we should look into this.
OTOH: nothing wrong with calling
posix_fallocate()if you need the space guarantees it provides for proper operation...
Right, but it's something that the filesystem has no real control over. We've been asked to allocate blocks immediately by fallocate(), and so we get what we get....
Cheers,
Dave.
Signature · 7 lines
Dave Chinner [email protected] _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
Quoted 3 lines · Florian Weimer
I see rather dramatic fragmentation of the systemd journal when it is stored on XFS, and it calls
posix_fallocatebefore writing data to the file.
You mean it calls fallocate before each write? That's not very useful behaviour and should be fixed. If it calls fallocate for the whole expeted file size (or large increments) it should not fragment the file, and if it does there's a bug we'd need to look into.
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