dear developers, can we have notdatacow + checksumming, plz?

I've had some discussions on the list these days about not having checksumming with nodatacow (mostly with Hugo and Duncan).

They both basically told me it wouldn't be straight possible with CoW, and Duncan thinks it may not be so much necessary, but none of them could give me really hard arguments, why it cannot work (or perhaps I was just too stupid to understand them ^^)... while at the same time I think that it would be generally utmost important to have checksumming (real world examples below).

My understanding of BTRFS is that the metadata referencing data blocks has the
checksums for those blocks, then the blocks which link to that metadata (EG
directory entries referencing file metadata) has checksums of those.

For each
metadata block there is a new version that is eventually linked from a new
version of the tree root.

This means that the regular checksum mechanisms can't work with nocow data.  A
filesystem can have checksums just pointing to data blocks but you need to
cater for the case where a corrupt metadata block points to an old version of
a data block and matching checksum.  The way that BTRFS works with an entire
checksumed tree means that there's no possibility of pointing to an old
version of a data block.

The NetApp published research into hard drive errors indicates that they are
usually in small numbers and located in small areas of the disk.  So if BTRFS
had a nocow file with any storage method other than dup you would have metadata
and file data far enough apart that they are not likely to be hit by the same
corruption (and the same thing would apply with most Ext4 Inode tables and
data blocks).

I think that a file mode where there were checksums on data blocks with no checksums on the metadata tree would be useful.  But it would
require a moderate amount of coding

and there's lots of other things that the
developers are working on.

(consider that question being asked with that face on: http://goo.gl/LQaOuA)

Hey.

I've had some discussions on the list these days about not having checksumming with nodatacow (mostly with Hugo and Duncan).

They both basically told me it wouldn't be straight possible with CoW, and Duncan thinks it may not be so much necessary, but none of them could give me really hard arguments, why it cannot work (or perhaps I was just too stupid to understand them ^^)... while at the same time I think that it would be generally utmost important to have checksumming (real world examples below).

Also, I remember that in 2014, Ted Ts'o told me that there are some plans ongoing to get data checksumming into ext4, with possibly even some guy at RH actually doing it sooner or later.

Since these threads were rather admin-work-centric, developers may have skipped it, therefore, I decided to write down some thoughts&ideas label them with a more attracting subject and give it some bigger attention.
O:-)

1) Motivation why, it makes sense to have checksumming (especially also in the nodatacow case)

I think of all major btrfs features I know of (apart from the CoW itself and having things like reflinks), checksumming is perhaps the one that distinguishes it the most from traditional filesystems.

Sure we have snapshots, multi-device support and compression - but we could have had that as well with LVM and software/hardware RAID... (and ntfs supported compression IIRC ;) ).
Of course, btrfs does all that in a much smarter way, I know, but it's nothing generally new.
The *data* checksumming at filesystem level, to my knowledge, is however. Especially that it's always verified. Awesome. :-)

When one starts to get a bit deeper into btrfs (from the admin/end-user side) one sooner or later stumbles across the recommendation/need to use nodatacow for certain types of data (DBs, VM images, etc.) and the reason, AFAIU, being the inherent fragmentation that comes along with the CoW, which is especially noticeable for those types of files with lots of random internal writes.

Now duncan implied, that this could improve in the future, with the auto-defragmentation getting (even) better, defrag becoming usable again for those that do snapshots or reflinked copies and btrfs itself generally maturing more and more.
But I kinda wonder to what extent one will be really able to solve that, what seems to me a CoW-inherent "problem",...
Even *if* one can make the auto-defrag much smarter, it would still mean that such files, like big DBs, VMs, or scientific datasets that are internally rewritten, may get more or less constantly defragmented. That may be quite undesired...
a) for performance reasons (when I consider our research software which often has IO as the limiting factor and where we want as much IO being used by actual programs as possible)...

b) SSDs...
Not really sure about that; btrfs seems to enable the autodefrag even when an SSD is detected,... what is it doing? Placing the block in a smart way on different chips so that accesses can be better parallelised by the controller?

Anyway, (a) is could be already argument enough, not to run solve the problem by a smart-[auto-]defrag, should that actually be implemented.

So I think having notdatacow is great and not just a workaround till everything else gets better to handle these cases.
Thus, checksumming, which is such a vital feature, should also be possible for that.

Duncan also mention that in some of those cases, the integrity is already protected by the application layer, making it less important to have it at the fs-layer.
Well, this may be true for file-sharing protocols, but I wouldn't know that relational DBs really do cheksuming of the data.

They have journals, of course, but these protect against crashes, not against silent block errors and that like.
And I wouldn't know that VM hypervisors do checksuming (but perhaps I've just missed that).

Here I can give a real-world example, from the Tier-2 that I run for LHC at work/university.
We have large amounts of storage (perhaps not as large as what Google and Facebook have, or what the NSA stores about us)... but it's still some ~ 2PiB, or a bit more.
That's managed with some special storage management software called dCache. dCache even stores checksums, but per file, so that means for normal reads, these cannot be verified (well technically it's supported, but with our usual file sizes, this is not working) so what remains are scrubs.
For The two PiB, we have some... roughly 50-60 nodes, each with something between 12 and 24 disks, usually in either one or two RAID6 volumes, all different kinds of hard disks.
And we do run these scrubs quite rarely, since it costs IO that could be used for actual computing jobs (a problem that wouldn't be there with how btrfs calculates the sums on read, the data is then read anyway)... so likely there are even more errors that are just never noticed, because the datasets are removed again, before being scrubbed.

Long story short, it does happen every now and then, that a scrub shows file errors, for neither the RAID was broken, nor there were any block errors reported by the disks, or anything suspicious in SMART. In other words, silent block corruption.

One may rely on the applications to do integrity protection, but I think that's not realistic, and perhaps that shouldn't be their task anyway (at least not when it's about storage device block errors and that like).

I don't think it's on the horizon that things like DBs or large scientific data files do their own integrity protection (i.e. one that protects against bad blocks, and not just journalling that preserves consistency in case of crashes).

And handling that on the fs level is anyway quite nice, I think. It doesn't mean that countless applications need to handle this on the application layer, making it configurable whether it should be enabled (for integrity protection) or disabled (for more speed), each of them writing a lot of code for that.
If we can control that on the fs layer, by setting datasum/nodatasum, all needed is already there - except, that as of now, nodatacowed stuff is excluded in btrfs.

2) Technical

Okay the following is obviously based on my naive view of how things could work, which may not necessarily go well with how an actual fs developer sees things ;-)

As said in the introduction, I can't quite believe that data checksumming should in principle be possible for ext4, but not for btrfs non-CoWed parts.

Duncan&Hugo said, the reason is basically it cannot do checksums with no-CoW, because there's no guarantee that the fs doesn't end up inconsistently...

But, AFAIU, not doing CoW, while not having a journal (or does it have one for these cases???) almost certainly means that the data (not necessarily the fs) will be inconsistent in case of a crash during a no-CoWed write anyway, right?
Wouldn't it be basically like ext2?

Or we have the case of multi-device, e.g. RAID1, multiple copies of the same blocks, a crash has happened during writing such (no-CoWed and no- checksummed)...
Again it's almost certainly that at least one (maybe even both) of the blocks contains garbage and likely (at least a 50% chance) we get that one when the actual read happens later (I was told btrfs would behave in these cases like e.g MD RAID does,... deliver what the first readable block said).

If btrfs would calculate checksums and write them e.g. after or before the actual data was written,... what would be the worst that could happen (in my naive understanding of course ;-) ) at a crash?
- I'd say either one is lucky, and checksum and data matches. Yay.
- Or it doesn't match, which could boil down to the following two cases:
- the data wasn't written out correctly and is actually garbage => then we can be happy, that the checksum wouldn't match and we'd get an error
- the data was written out correctly, but before the csum was written the system crashed, so the csum would now tell us that the block is bad, while in reality it isn't.
or the other way round:
the csum was written out (completely)... and no data was written

at all before the system crashed (so the old block would be still completely there)
=> in both cases: so what? Having that particular case happening
is probably far less likely, than csumming actually detecting a bad block, or not completely written data in case of a crash. (Not to talk about all the cases where nothing crashes, and where we simply would want to detect block errors, bus errors, etc.)

=> Of course it wouldn't be as nice as in CoW, where it could
simply take the most recent consistent state of that block, but still way better than:
- delivering bogus data to the application in n other cases
- not being able to decide which of m block copies is valid, if a RAID is scrubbed

And as said before, AFAIU, nodatacow'ed files have no journal in btrfs as in ext3/4, so it's basically anyway that such files, when written during a crash, may end up in any state, right? Which makes not having a csum sound even worse, since nothing tells that this file is possibly bad.

Not having checksumming seems to be especially bad in the multi-device case... what happens when one runs a scrub? AFAIU, it simply does what e.g. MD does: taking the first readable block, writing it to any others, thereby possibly destroying the actually good one?

Not sure about whether the following would make any practical sense:
If data checksumming would work for nodatacow, then maybe some people may even choose to run btrfs in CoW1 mode,.. they still could have most fancy features from btrfs (checksumming, snapshots, perhaps even refcopy?) but unless snapshots or refcopies are explicitly made, btrfs doesn't do CoW.

Quoted 11 lines

When one starts to get a bit deeper into btrfs (from the admin/end- user
side) one sooner or later stumbles across the recommendation/need to
use nodatacow for certain types of data (DBs, VM images, etc.) and the
reason, AFAIU, being the inherent fragmentation that comes along with
the CoW, which is especially noticeable for those types of files with
lots of random internal writes.

It is worth pointing out that in the case of DB's at least, this is because at least some of the do COW internally to provide the transactional semantics that are required for many workloads.

Quoted 5 lines

a) for performance reasons (when I consider our research software which
often has IO as the limiting factor and where we want as much IO being
used by actual programs as possible)...

There are other things that can be done to improve this.  I would assume
of course that you're already doing some of them (stuff like using dedicated storage controller cards instead of the stuff on the motherboard), but some things often get overlooked, like actually taking
the time to fine-tune the I/O scheduler for the workload (Linux has particularly brain-dead default settings for CFQ, and the deadline I/O
scheduler is only good in hard-real-time usage or on small hard drives
that actually use spinning disks).

The big argument for defragmenting a SSD is that it makes it such that
you require fewer I/O requests to the device to read a file

The problem is not entirely the lack of COW semantics, it's also the fact that it's impossible to implement an atomic write on a hard disk.

Quoted 2 lines

but I wouldn't know that relational DBs really do cheksuming of the data.

All the ones I know of except GDBM and BerkDB do in fact provide the option of checksumming.  It's pretty much mandatory if you want to be considered for usage in financial, military, or medical applications.

Quoted 6 lines

Long story short, it does happen every now and then, that a scrub shows
file errors, for neither the RAID was broken, nor there were any block
errors reported by the disks, or anything suspicious in SMART. In other words, silent block corruption.

Or a transient error in system RAM that ECC didn't catch, or a undetected error in the physical link layer to the disks, or an error in
the disk cache or controller, or any number of other things.

  BTRFS
could only protect against some cases, not all (for example, if you have
a big enough error in RAM that ECC doesn't catch it, you've got serious
issues that just about nothing short of a cold reboot can save you from).

Quoted 7 lines

But, AFAIU, not doing CoW, while not having a journal (or does it have
one for these cases???) almost certainly means that the data (not necessarily the fs) will be inconsistent in case of a crash during a
no-CoWed write anyway, right?
Wouldn't it be basically like ext2?

Kind of, but not quite.  Even with nodatacow, metadata is still COW, which is functionally as safe as a traditional journaling filesystem like XFS or ext4.

Absolute worst case scenario for both nodatacow on
BTRFS, and a traditional journaling filesystem, the contents of the file
are inconsistent.  However, almost all of the things that are recommended use cases for nodatacow (primarily database files and VM images) have some internal method of detecting and dealing with corruption (because of the traditional filesystem semantics ensuring metadata consistency, but not data consistency).

Quoted 4 lines

   - the data was written out correctly, but before the csum was written the system crashed, so the csum would now tell us that the
     block is bad, while in reality it isn't.

There is another case to consider, the data got written out, but the crash happened while writing the checksum (so the checksum was partially
written, and is corrupt).  This means we get a false positive on a disk
error that isn't there, even when the data is correct, and that should
be avoided if at all possible.

Also, because of how disks work, and the internal layout of BTRFS, it's
a lot more likely than you think that the data would be written but the
checksum wouldn't.  The checksum isn't part of the data block, nor is it
stored with it, it's actually a part of the metadata block that stores
the layout of the data for that file on disk.

Because of the nature of
the stuff that nodatacow is supposed to be used for, it's almost always
better to return bad data than it is to return no data (if you can get
any data, then it's usually possible to recover the database file or VM
image, but if you get none, it's a lot harder to recover the file).

Quoted 7 lines

=> Of course it wouldn't be as nice as in CoW, where it could simply take the most recent consistent state of that block, but still way better than:
    - delivering bogus data to the application in n other cases
    - not being able to decide which of m block copies is valid, if a
      RAID is scrubbed

This gets _really_ scarily dangerous for a RAID setup, because we _absolutely_ can't ensure consistency between disks without using COW.

As of right now, we dispatch writes to disks one at a time (although this would still be just as dangerous even if we dispatched writes in parallel)

so if we crash it's possible that one disk would hold the old data, one would hold the new data

and _both_ would have correct
checksums, which means that we would non-deterministically return one block or the other when an application tries to read it, and which block
we return could change _each_ time the read is attempted, which absolutely breaks the semantics required of a filesystem on any modern
OS (namely, the file won't change unless something writes to it).

As I stated above, most of the stuff that nodatacow is intended for already has it's own built-in protection.  No self-respecting RDBMS would be caught dead without internal consistency checks, and they all
do COW internally anyway (because it's required for atomic transactions,

which are an absolute requirement for database systems), and in fact that's part of why performance is so horrible for them on a COW filesystem.  As far as VM's go, either the disk image should have it's
own internal consistency checks (for example, qcow2 format, used by QEMU, which also does COW internally), or the guest OS should have such
checks.

Quoted 11 lines · Austin S. Hemmelgarn

Quoted 11 lines

When one starts to get a bit deeper into btrfs (from the admin/end- user
side) one sooner or later stumbles across the recommendation/need to
use nodatacow for certain types of data (DBs, VM images, etc.) and the
reason, AFAIU, being the inherent fragmentation that comes along with
the CoW, which is especially noticeable for those types of files with
lots of random internal writes.

Quoted 3 lines

It is worth pointing out that in the case of DB's at least, this is because at least some of the do COW internally to provide the transactional semantics that are required for many workloads.

Guess that also applies to some VM images then, IIRC qcow2 does CoW.

Quoted 8 lines

Quoted 8 lines

a) for performance reasons (when I consider our research software which
often has IO as the limiting factor and where we want as much IO being
used by actual programs as possible)...

Quoted 12 lines

There are other things that can be done to improve this. I would assume
of course that you're already doing some of them (stuff like using dedicated storage controller cards instead of the stuff on the motherboard), but some things often get overlooked, like actually taking
the time to fine-tune the I/O scheduler for the workload (Linux has particularly brain-dead default settings for CFQ, and the deadline I/O
scheduler is only good in hard-real-time usage or on small hard drives
that actually use spinning disks).

Well sure, I think we'de done most of this and have dedicated controllers, at least of a quality that funding allows us ;-) But regardless how much one tunes, and how good the hardware is. If you'd then loose always a fraction of your overall IO, and be it just 5%, to defragging these types of files, one may actually want to avoid this at all, for which nodatacow seems *the* solution.

Quoted 5 lines

The big argument for defragmenting a SSD is that it makes it such that
you require fewer I/O requests to the device to read a file

I've had read about that too, but since I haven't had much personal experience or measurements in that respect, I didn't list it :)

Quoted 4 lines

The problem is not entirely the lack of COW semantics, it's also the fact that it's impossible to implement an atomic write on a hard disk.

Sure... but that's just the same for the nodatacow writes of data. (And the same, AFAIU, for CoW itself, just that we'd notice any corruption in case of a crash due to the CoWed nature of the fs and could go back to the last generation).

Quoted 7 lines

Quoted 4 lines

but I wouldn't know that relational DBs really do cheksuming of the data.

All the ones I know of except GDBM and BerkDB do in fact provide the option of checksumming. It's pretty much mandatory if you want to be considered for usage in financial, military, or medical applications.

Hmm I see... PostgreSQL seem to have it since 9.3 ... didn't know that... only crc16 but at least something.

Quoted 10 lines

Quoted 6 lines

Long story short, it does happen every now and then, that a scrub shows
file errors, for neither the RAID was broken, nor there were any block
errors reported by the disks, or anything suspicious in SMART. In other words, silent block corruption.

Or a transient error in system RAM that ECC didn't catch, or a undetected error in the physical link layer to the disks, or an error in
the disk cache or controller, or any number of other things.

Well sure,... I was referring to these particular cases, where silent block corruption was the most likely reason.
The data was reproducibly read identical, which probably rules out bad RAM or controller, etc.

Quoted 7 lines

BTRFS
could only protect against some cases, not all (for example, if you have
a big enough error in RAM that ECC doesn't catch it, you've got serious
issues that just about nothing short of a cold reboot can save you from).

Sure, I haven't claimed, that checksumming for no-CoWed data is a solution for everything.

Quoted 10 lines

Quoted 7 lines

But, AFAIU, not doing CoW, while not having a journal (or does it have
one for these cases???) almost certainly means that the data (not necessarily the fs) will be inconsistent in case of a crash during a
no-CoWed write anyway, right?
Wouldn't it be basically like ext2?

Kind of, but not quite. Even with nodatacow, metadata is still COW, which is functionally as safe as a traditional journaling filesystem like XFS or ext4.

Sure, I was referring to the data part only, should have made that more clear.

Quoted 8 lines

Absolute worst case scenario for both nodatacow on
BTRFS, and a traditional journaling filesystem, the contents of the file
are inconsistent. However, almost all of the things that are recommended use cases for nodatacow (primarily database files and VM images) have some internal method of detecting and dealing with corruption (because of the traditional filesystem semantics ensuring metadata consistency, but not data consistency).

What about VMs? At least a quick google search didn't give me any results on whether there would be e.g. checksumming support for qcow2. For raw images there surely is not.

And even if DBs do some checksumming now, it may be just a consequence of that missing in the filesystems.
As I've written somewhere else in the previous mail: it's IMHO much better if one system takes care on this, where the code is well tested, than each application doing it's own thing.

Quoted 6 lines

Quoted 6 lines

- the data was written out correctly, but before the csum was written the system crashed, so the csum would now tell us that
the
block is bad, while in reality it isn't.

Quoted 8 lines

There is another case to consider, the data got written out, but the crash happened while writing the checksum (so the checksum was partially
written, and is corrupt). This means we get a false positive on a disk
error that isn't there, even when the data is correct, and that should
be avoided if at all possible.

I've had that, and I've left it quoted above.
But as I've said before: That's one case out of many? How likely is it that the crash happens exactly after a large data block has been written followed by a relatively tiny amount of checksum data. I'd assume it's far more likely that the crash happens during writing the data.

And regarding "reporting data to be in error, which is actually correct"... isn't that what all journaling systems may do?

And, AFAIU, isn't that also what can happen in btrfs? The data was already CoWed, but the metadata wasn't written out... so it would fall back somehow - here's where the unicorn[0] does it's job - to an older generation?

So that would be nothing really new.

Quoted 9 lines

Also, because of how disks work, and the internal layout of BTRFS, it's
a lot more likely than you think that the data would be written but the
checksum wouldn't. The checksum isn't part of the data block, nor is it
stored with it, it's actually a part of the metadata block that stores
the layout of the data for that file on disk.

Well it was clear to me, that data+csum isn't sequentially on disk are there any numbers from real studies how often it would happen that data is written correctly but not the metadata?
And even if such study would show that - crash isn't the only problem we want to protect here (silent block errors, bus errors, etc). I don't want to say crashes never happen, but in my practical experience they don't happen that often either,...

Losing a few blocks of valid data in the rare case of crashes, seems to be a penalty worth, when one gains confidence in data integrity in all others.

Quoted 10 lines

Because of the nature of
the stuff that nodatacow is supposed to be used for, it's almost always
better to return bad data than it is to return no data (if you can get
any data, then it's usually possible to recover the database file or VM
image, but if you get none, it's a lot harder to recover the file).

No. Simply no! :D

Seriously:
If you have bad data, for whichever reason (crash, silent block errors, etc.), it's always best to notice.
*Then* you can decide what to do:
- Is there a backup and does one want to get the data from that backup, rather than continuing to use bad data, possibly even overwriting good backups one week later
- Is there either no backup or the effort of recovering it is to big and the corruption doesn't matter enough (e.g. when you have large video files, and there is a sinlge bit flip... well that may just mean that one colour looks a tiny bit different)

But that's nothing the fs could or should decide for the user.

After I've had sent the initial mail from this thread I remembered what I've had forgotten to add:
Is there a way in btrfs, to tell it that gives clearance to a file which it found to be in error based on checksums?

Cause *this* is IMHO the proper solution for your "it's almost always better to return bad data than it is to return no data".

When we at the Tier-2 detect a file error that we cannot correct by means of replicas, we determine the owner of that file, tell him about the issue, and if he wants to continue using the broken file, there's a way in the storage management system to rewrite the checksum.

Quoted 10 lines

Quoted 7 lines

=> Of course it wouldn't be as nice as in CoW, where it could
simply take the most recent consistent state of that block, but still way better than:
- delivering bogus data to the application in n other cases
- not being able to decide which of m block copies is valid, if
a
RAID is scrubbed

This gets _really_ scarily dangerous for a RAID setup, because we _absolutely_ can't ensure consistency between disks without using COW.

Hmm now I just thought "damn he got me" ;-)

Quoted 3 lines

As of right now, we dispatch writes to disks one at a time (although this would still be just as dangerous even if we dispatched writes in parallel)

Sure...

Quoted 2 lines

so if we crash it's possible that one disk would hold the old data, one would hold the new data

sure..

Quoted 8 lines

and _both_ would have correct
checksums, which means that we would non-deterministically return one block or the other when an application tries to read it, and which block
we return could change _each_ time the read is attempted, which absolutely breaks the semantics required of a filesystem on any modern
OS (namely, the file won't change unless something writes to it).

Here I do not longer follow you, so perhaps you (or someone else) can explain a bit further. :-)

a) Are checksums really stored per device (and not just once in the metadata? At least from my naive understanding this would either mean that there's a waste of storage, or that the csums are made on data that could vary from device to device (e.g. the same data split up in different extents, or compression on one device but not on the other). but..

b) that problem (different data each with valid corresponding csums) should in principle exist for CoWed data as well, right? And there, I guess, it's solved by CoWing the metadata... (which would still be the case for no-dataCoWed files).

Don't know what btrfs does in the CoWed case when such incident happens... how does it decide which of two such corresponding blocks would be the newer one? The generations?

Anyway, since metadata would still be CoWed, I think I may have gotten once again out of the tight spot - at least until you explain me, why my naive understanding, as laid out just above, doesn't work out O:-)

Quoted 16 lines

As I stated above, most of the stuff that nodatacow is intended for already has it's own built-in protection. No self-respecting RDBMS would be caught dead without internal consistency checks, and they all
do COW internally anyway (because it's required for atomic transactions,
which are an absolute requirement for database systems), and in fact that's part of why performance is so horrible for them on a COW filesystem. As far as VM's go, either the disk image should have it's
own internal consistency checks (for example, qcow2 format, used by QEMU, which also does COW internally), or the guest OS should have such
checks.

Well, for PostgreSQL it's still fairly new (9.3, as I've said above, ht tps://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.3#Data_Chec ksums), but it's not done per default (http://www.postgresql.org/docs/c
urrent/static/app-initdb.html), and they warn about a noticable performance benefit (though I have of course no data whether this would be better/similar/worse to what is implied by btrfs checksumming).

I've tried to find something for MySQL/MariaDB, but the only thing I could find there was: CHECKSUM TABLE
But that seems to be a SQL command, i.e. not on-read checksumming as we're talking about, but rather something the application/admin would need to do manually.

BDB seems to support it (https://docs.oracle.com/cd/E17076_04/html/api_
reference/C/dbset_flags.html), but again not per default. (And yes, we have quite big ones of them ^^)

SQLite doesn't seem to do it, at least not per default? (https://www.sq
lite.org/fileformat.html)

I tried once again to find any reference that qcow2 (which alone I think would justify having csum support for nodatacow) supports checksumming.
https://people.gnome.org/~markmc/qcow-image-format.html which seems to be the original definition, doesn't tell[1] anything about it. raw image, do of course not to any form of checksumming... I had a short glance at OVF, but nothing popped up immediately that would make me believe it supports checksumming.
Well there's VDI and VHD left... but are these still used seriously? I guess KVM and Xen people mostly use raw or qcow2 these days, don't they?

So given all that, the picture looks a bit different again, I think. None of major FLOSS DBs doesn't do any checksumming per default, MySQL doesn't seem to support it, AFAICT. No VM image format seems to even support it.

And not to talk about countless of scientific data formats, which are mostly not widely known to the FLOSS world, but which are used with FLOSS software/Linux.

So AFAICT, the only thing left is torrent/edonkey files. And do these store the checksums along the files? Or do they rather wait until a chunk has been received, verify that and then throw it away?
In any case however, at least some of these files types eventually end up in the raw files, without any checksum (as that's only used during download),... so when the files remain in the nodatacow area, they're again at risk (+ during the time after the P2P software has finally committed them to disk, and they'd be moved to CoWed and thus checksummed areas)

[0] http://abstrusegoose.com/120
[1] admittedly I just cross read over it, and searched for the usual suspect strings (hash, crc, sum) ;)

And in particular, the only
journaling filesystem that I know of that even allows the option of journaling the file contents instead of just metadata is ext4.

AFAIUI, checksums are stored per-instance for every block. This is important in a multi-device filesystem in case you lose a device, so that you still have a checksum for the block. There should be no difference between extent layout and compression between devices however.

Quoted 8 lines

Well sure, I think we'de done most of this and have dedicated controllers, at least of a quality that funding allows us ;-) But regardless how much one tunes, and how good the hardware is. If you'd then loose always a fraction of your overall IO, and be it just
5%, to defragging these types of files, one may actually want to avoid
this at all, for which nodatacow seems *the* solution.

nodatacow only works for that if the file is pre-allocated, if it isn't,
then it still ends up fragmented.

Quoted 4 lines

Quoted 4 lines

The problem is not entirely the lack of COW semantics, it's also the
fact that it's impossible to implement an atomic write on a hard disk.

Quoted 4 lines

Sure... but that's just the same for the nodatacow writes of data. (And the same, AFAIU, for CoW itself, just that we'd notice any corruption in case of a crash due to the CoWed nature of the fs and could go back to the last generation).

Yes, but it's also the reason that using either COW or a log- structured
filesystem (like NILFS2, LogFS, or I think F2FS) is important for consistency.

Quoted 4 lines

What about VMs? At least a quick google search didn't give me any results on whether there would be e.g. checksumming support for qcow2.
For raw images there surely is not.

I don't mean that the VMM does checksumming, I mean that the guest OS should be the one to handle the corruption.  No sane OS doesn't run at
least some form of consistency checks when mounting a filesystem.

Quoted 8 lines

And even if DBs do some checksumming now, it may be just a consequence
of that missing in the filesystems.
As I've written somewhere else in the previous mail: it's IMHO much better if one system takes care on this, where the code is well tested,
than each application doing it's own thing.

That's really a subjective opinion.  The application knows better than
we do what type of data integrity it needs, and can almost certainly do
a better job of providing it than we can.

Quoted 16 lines

Quoted 6 lines

Quoted 6 lines

    - the data was written out correctly, but before the csum was
      written the system crashed, so the csum would now tell us that
the
      block is bad, while in reality it isn't.

Quoted 10 lines

There is another case to consider, the data got written out, but the
crash happened while writing the checksum (so the checksum was partially
written, and is corrupt).  This means we get a false positive on a
disk
error that isn't there, even when the data is correct, and that should
be avoided if at all possible.

Quoted 8 lines

I've had that, and I've left it quoted above.
But as I've said before: That's one case out of many? How likely is it
that the crash happens exactly after a large data block has been written followed by a relatively tiny amount of checksum data. I'd assume it's far more likely that the crash happens during writing
the data.

Except that the whole metadata block pointing to that data block gets rewritten, not just the checksum.

Quoted 2 lines

And regarding "reporting data to be in error, which is actually correct"... isn't that what all journaling systems may do?

No, most of them don't actually do that.  The general design of a journaling filesystem is that the journal is used as what's called a Write-Intent-Log (WIL), the purpose of which is to say 'Hey, I'm going
to write this data here in a little while.' so that when your system dies while writing that data, you can then finish writing it correctly
when the system gets booted up again.  And in particular, the only journaling filesystem that I know of that even allows the option of journaling the file contents instead of just metadata is ext4.

Quoted 6 lines

And, AFAIU, isn't that also what can happen in btrfs? The data was already CoWed, but the metadata wasn't written out... so it would fall
back somehow - here's where the unicorn[0] does it's job - to an older
generation?

Kind of, there are some really rare cases where it's possible if you get
_really_ unlucky on a multi-device filesystem that things get corrupted
such that the filesystem thinks that data that is perfectly correct is
invalid, and thinks that the other copy which is corrupted is valid. (I've actually had this happen before, it was not fun trying to recover
from it).

Quoted 11 lines

Well it was clear to me, that data+csum isn't sequentially on disk are
there any numbers from real studies how often it would happen that data
is written correctly but not the metadata?
And even if such study would show that - crash isn't the only problem
we want to protect here (silent block errors, bus errors, etc). I don't want to say crashes never happen, but in my practical experience they don't happen that often either,...

Quoted 5 lines

Losing a few blocks of valid data in the rare case of crashes, seems to
be a penalty worth, when one gains confidence in data integrity in all
others.

That _really_ depends on what the data is.  If you made that argument to
the IT department at a financial institution, they would probably fall
over laughing at you.

Quoted 1 line

But that's nothing the fs could or should decide for the user.

OK, good point about this being policy.  And in some cases (executables,
configuration for administrative software, similar things), it is better
to just return an error, but in many cases, that's not what most desktop
users would want.  Think document files, where a single byte error could
easily be corrected by the user, or configuration files for sanely written apps (It's a lot nicer (and less confusing for someone without a

lot of low-level computer background) to say 'Hey, your configuration file is messed up, here's how to fix it', than it is to say 'Hey, I couldn't read your configuration file').  And because BTRFS is supposed
to be a general purpose filesystem, it has to account for the case of desktop users, and because server admins are supposed to be smart, the
default should be for desktop usage.

Quoted 9 lines

a) Are checksums really stored per device (and not just once in the metadata? At least from my naive understanding this would either mean
that there's a waste of storage, or that the csums are made on data that could vary from device to device (e.g. the same data split up in
different extents, or compression on one device but not on the other).
but..

AFAIUI, checksums are stored per-instance for every block.  This is important in a multi-device filesystem in case you lose a device, so that you still have a checksum for the block.  There should be no difference between extent layout and compression between devices however.

Quoted 7 lines

b) that problem (different data each with valid corresponding csums)
should in principle exist for CoWed data as well, right? And there, I
guess, it's solved by CoWing the metadata... (which would still be the
case for no-dataCoWed files).

Yes.

Quoted 4 lines

Don't know what btrfs does in the CoWed case when such incident happens... how does it decide which of two such corresponding blocks
would be the newer one? The generations?

Usually, but like I mentioned above there are edge cases that can occur
as a result of data corruption on disk or other really rare circumstances.  In the particular case of multiple copies of a block with different data but valid checksums, I'm about 95% certain that it
will non-deterministically return one block or the other on an arbitrary
read when the read doesn't hit the VFS cache.

  This is a potential issue
for COW as well, but much less likely because it can more easily detect
the corruption and fix it.

Quoted 7 lines

Anyway, since metadata would still be CoWed, I think I may have gotten
once again out of the tight spot - at least until you explain me, why
my naive understanding, as laid out just above, doesn't work out O:-)

Hmm, I had forgotten about the metadata being COW, that does avoid the
situation above under the specified circumstances, but does not avoid it
happening due to disk errors (although that's extremely unlikely,a s it
would require direct correlation of the errors in a way that is statistically impossible).

Quoted 11 lines

Well, for PostgreSQL it's still fairly new (9.3, as I've said above, ht tps://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.3#Data_ Chec
ksums), but it's not done per default (http://www.postgresql.org/do
cs/c
urrent/static/app-initdb.html), and they warn about a noticable performance benefit (though I have of course no data whether this would
be better/similar/worse to what is implied by btrfs checksumming).

Quoted 8 lines

I've tried to find something for MySQL/MariaDB, but the only thing I
could find there was: CHECKSUM TABLE
But that seems to be a SQL command, i.e. not on-read checksumming as
we're talking about, but rather something the application/admin would
need to do manually.

I actually had been referring to this, with the assumption that the application would use it to verify it's own data.  I hadn't realized PostgreSQL had in-line support for it.

VDI is still widely used, because it's the default for Virtual Box when
creating a VM.

  VHD is way more widely used than it should be, solely because there are insane people out there using Windows as a virtualization host.  You also forgot VMDK, which is what VMWare uses almost exclusively, but I don't think it has built-in checksumming.

As for Xen, the BCP are to avoid using image files like the plague, and
use disks directly instead (or more commonly, use either LVM, or ZFS with zvols).

Quoted 7 lines

So given all that, the picture looks a bit different again, I think.
None of major FLOSS DBs doesn't do any checksumming per default, MySQL
doesn't seem to support it, AFAICT. No VM image format seems to even
support it.

Again, most of my intent in referring to those was that the application
or the Guest OS would do the verification itself.

If the application doesn't have that type of thing built in, then that's
not something the filesystem should be worrying about, that's the job of
the application developers to deal with.

The point of a filesystem is
to store data within the integrity guarantees provided by the hardware,
possibly with some additional protection

In the case of stuff like torrents and such, all the good software for
working with them has an option to verify the file after downloading.

Quoted 8 lines · Austin S. Hemmelgarn

Quoted 8 lines

Well sure, I think we'de done most of this and have dedicated controllers, at least of a quality that funding allows us ;-) But regardless how much one tunes, and how good the hardware is. If you'd then loose always a fraction of your overall IO, and be it just
5%, to defragging these types of files, one may actually want to avoid
this at all, for which nodatacow seems *the* solution.

Quoted 3 lines

nodatacow only works for that if the file is pre-allocated, if it isn't,
then it still ends up fragmented.

Hmm is that "it may end up fragmented" or a "it will definitely? Cause I'd have hoped, that if nothing else had been written in the meantime, btrfs would perhaps try to write next to the already allocated blocks.

Quoted 6 lines

Quoted 6 lines

Quoted 6 lines

The problem is not entirely the lack of COW semantics, it's also the
fact that it's impossible to implement an atomic write on a hard disk.

Quoted 8 lines

Quoted 4 lines

Sure... but that's just the same for the nodatacow writes of data. (And the same, AFAIU, for CoW itself, just that we'd notice any corruption in case of a crash due to the CoWed nature of the fs and could go back to the last generation).

Yes, but it's also the reason that using either COW or a log- structured
filesystem (like NILFS2, LogFS, or I think F2FS) is important for consistency.

So then it's no reason why it shouldn't work.
The meta-data is CoWed, any incomplete writes of checksumdata in that (be it for CoWed data or no-CoWed data, should the later be implemented), would be protected at that level.

Currently, the no-CoWed data is, AFAIU completely at risk of being corrupted (no checksums, no journal).

Checksums on no-CoWed data would just improve that.

Quoted 10 lines

Quoted 6 lines

What about VMs? At least a quick google search didn't give me any results on whether there would be e.g. checksumming support for qcow2.
For raw images there surely is not.

I don't mean that the VMM does checksumming, I mean that the guest OS should be the one to handle the corruption. No sane OS doesn't run at
least some form of consistency checks when mounting a filesystem.

Well but we're not talking about having a filesystem that "looks clear" here. For this alone we wouldn't need any checksumming at all.

We talk about data integrity protection, i.e. all files and their contents. Nothing which a fsck inside a guest VM would ever notice (I mean by a fsck), if there are just some bit flips or things like that.

Quoted 10 lines

Quoted 10 lines

And even if DBs do some checksumming now, it may be just a consequence
of that missing in the filesystems.
As I've written somewhere else in the previous mail: it's IMHO much better if one system takes care on this, where the code is well tested,
than each application doing it's own thing.

Quoted 5 lines

That's really a subjective opinion. The application knows better than
we do what type of data integrity it needs, and can almost certainly do
a better job of providing it than we can.

Hmm I don't see that.
When we, at the filesystem level, provide data integrity, than all data is guaranteed to be valid.
What more should an application be able to provide? At best they can do the same thing faster, but even for that I see no immediate reason to believe it.

And in practise it seems far more likely that if countless applications should such task on their own, that it's more error prone (that's why we have libraries for all kinds of code, trying to reuse code, minimising the possibility of errors in countless home-brew solutions), or not done at all.

Quoted 26 lines

Quoted 26 lines

Quoted 8 lines

Quoted 8 lines

- the data was written out correctly, but before the csum
was
written the system crashed, so the csum would now tell us
that
the
block is bad, while in reality it isn't.

Quoted 10 lines

There is another case to consider, the data got written out, but the
crash happened while writing the checksum (so the checksum was partially
written, and is corrupt). This means we get a false positive on a
disk
error that isn't there, even when the data is correct, and that should
be avoided if at all possible.

I've had that, and I've left it quoted above.
But as I've said before: That's one case out of many? How likely is it
that the crash happens exactly after a large data block has been written followed by a relatively tiny amount of checksum data. I'd assume it's far more likely that the crash happens during writing
the data.

Quoted 2 lines

Except that the whole metadata block pointing to that data block gets rewritten, not just the checksum.

But that's the case anyway, isn't it? With or without checksums.

Quoted 5 lines

Quoted 5 lines

And regarding "reporting data to be in error, which is actually correct"... isn't that what all journaling systems may do?

Quoted 10 lines

No, most of them don't actually do that. The general design of a journaling filesystem is that the journal is used as what's called a Write-Intent-Log (WIL), the purpose of which is to say 'Hey, I'm going
to write this data here in a little while.' so that when your system dies while writing that data, you can then finish writing it correctly
when the system gets booted up again. And in particular, the only journaling filesystem that I know of that even allows the option of journaling the file contents instead of just metadata is ext4.

Well but that's just what I say... the system crashes,... the journal tells about anything that's not for sure cleanly on disk, even though it may have actually made it it.

Nothing more than what would happen in our case.

Quoted 6 lines

Quoted 6 lines

And, AFAIU, isn't that also what can happen in btrfs? The data was already CoWed, but the metadata wasn't written out... so it would fall
back somehow - here's where the unicorn[0] does it's job - to an older
generation?

Quoted 10 lines

Kind of, there are some really rare cases where it's possible if you get
_really_ unlucky on a multi-device filesystem that things get corrupted
such that the filesystem thinks that data that is perfectly correct is
invalid, and thinks that the other copy which is corrupted is valid. (I've actually had this happen before, it was not fun trying to recover
from it).

Doesn't really speak against nodatacow checksumming, AFAICS.

Quoted 18 lines

Quoted 13 lines

Well it was clear to me, that data+csum isn't sequentially on disk are
there any numbers from real studies how often it would happen that data
is written correctly but not the metadata?
And even if such study would show that - crash isn't the only problem
we want to protect here (silent block errors, bus errors, etc). I don't want to say crashes never happen, but in my practical experience they don't happen that often either,...

Quoted 5 lines

Losing a few blocks of valid data in the rare case of crashes, seems to
be a penalty worth, when one gains confidence in data integrity in all
others.

Quoted 5 lines

That _really_ depends on what the data is. If you made that argument to
the IT department at a financial institution, they would probably fall
over laughing at you.

Well but your point is completely moot, because for someone who cares so much in data, they wouldn't use nodatacow when btrfs has no journal and the data could end up in any state in case of crash.

And I'm quite certain that each financial institution rather clearly gets an error message (i.e. because the checksums don't very), after which they can get a backup, than having corrupt data taking for valid, and the debts of their customers being zeroed.

It's kinda strange how you argue against better integrity protection ;-)

Quoted 14 lines

Quoted 3 lines

But that's nothing the fs could or should decide for the user.

OK, good point about this being policy. And in some cases (executables,
configuration for administrative software, similar things), it is better
to just return an error, but in many cases, that's not what most desktop
users would want. Think document files, where a single byte error could
easily be corrected by the user, or configuration files for sanely written apps (It's a lot nicer (and less confusing for someone without a

Quoted 8 lines

lot of low-level computer background) to say 'Hey, your configuration file is messed up, here's how to fix it', than it is to say 'Hey, I couldn't read your configuration file'). And because BTRFS is supposed
to be a general purpose filesystem, it has to account for the case of desktop users, and because server admins are supposed to be smart, the
default should be for desktop usage.

Well but that's just the point I've made. The fs cannot decide what's better or not.
Your document could be an important config file that allows/disallows remote users access to resources. The single byte error could make a 0 to a 1, allowing world wide access.

It could be your thesis' data, or part of the document file, changing some numbers, which you won't easily notice but which makes everything bogus when examined.

I had brought the example with the video file, where it may not matter.

But in any case it's nothing what the fs can decide. The best it can do is give an error on read, and the tools to give clearance to such files (when they could not be auto-recovered by e.g. other copies).

All this is however only possible with checksumming.

Quoted 11 lines

Quoted 11 lines

a) Are checksums really stored per device (and not just once in the metadata? At least from my naive understanding this would either mean
that there's a waste of storage, or that the csums are made on data that could vary from device to device (e.g. the same data split up in
different extents, or compression on one device but not on the other).
but..

Quoted 5 lines

AFAIUI, checksums are stored per-instance for every block. This is important in a multi-device filesystem in case you lose a device, so that you still have a checksum for the block. There should be no difference between extent layout and compression between devices however.

hmm but if that's the case, especially the later, that the extents are the same on all devices,... then there's IMHO no need for data being stored per-instance (I guess you mean per device instance?) for every block.
The meta-data would have e.g. DUP anyway, so even if one device fails metadata would hopefully be still there.
And if metadata is coompletely lost, the fs is lost anyway, and csums don't matter anymore.

Quoted 14 lines

Quoted 9 lines

b) that problem (different data each with valid corresponding csums)
should in principle exist for CoWed data as well, right? And there, I
guess, it's solved by CoWing the metadata... (which would still be the
case for no-dataCoWed files).

Yes.

Quoted 4 lines

Don't know what btrfs does in the CoWed case when such incident happens... how does it decide which of two such corresponding blocks
would be the newer one? The generations?

Quoted 9 lines

Usually, but like I mentioned above there are edge cases that can occur
as a result of data corruption on disk or other really rare circumstances. In the particular case of multiple copies of a block with different data but valid checksums, I'm about 95% certain that it
will non-deterministically return one block or the other on an arbitrary
read when the read doesn't hit the VFS cache.

Hmm would be quite worrysome if that could happen, especially also in the CoW case.

Quoted 5 lines

This is a potential issue
for COW as well, but much less likely because it can more easily detect
the corruption and fix it.

But then again, there should be no difference to checksumming the no-CoWed data - the checksums would be CoWed again, if btrfs can detect it there, it should be fine.

Quoted 7 lines

Quoted 7 lines

Anyway, since metadata would still be CoWed, I think I may have gotten
once again out of the tight spot - at least until you explain me, why
my naive understanding, as laid out just above, doesn't work out O:-)

Quoted 8 lines

Hmm, I had forgotten about the metadata being COW, that does avoid the
situation above under the specified circumstances, but does not avoid it
happening due to disk errors (although that's extremely unlikely,a s it
would require direct correlation of the errors in a way that is statistically impossible).

Ah... here we go :-)

What exactly do you mean with disk errors here? IOW, what scenario do you think of, in which checksumming no-CoWed data could lead to any more corruptions than it can to without checksumming as well, or where any inconsistencies could get into the filesystem's meta-data, that couldn't already come in for checksummed+CoWed data and/or non- checksummed+CoWed data?

Quoted 20 lines

Quoted 12 lines

Well, for PostgreSQL it's still fairly new (9.3, as I've said above, ht tps://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.3#Data_ Chec
ksums), but it's not done per default (http://www.postgresql.org/do
cs/c
urrent/static/app-initdb.html), and they warn about a noticable performance benefit (though I have of course no data whether this would
be better/similar/worse to what is implied by btrfs checksumming).

Quoted 8 lines

I've tried to find something for MySQL/MariaDB, but the only thing I
could find there was: CHECKSUM TABLE
But that seems to be a SQL command, i.e. not on-read checksumming as
we're talking about, but rather something the application/admin would
need to do manually.

Quoted 3 lines

I actually had been referring to this, with the assumption that the application would use it to verify it's own data. I hadn't realized PostgreSQL had in-line support for it.

Well but the (fairly new) in-line support, is the only thing that we can really count here.

What mysql does is that it requires the app to do it.
a) It's likely that there are many apps which don't use this (maybe simply because they don't know it) and it's unlikely they'll all change.
While what we can do at the btrfs level (or what postgresql would do) works out of the box for everything.

b) I may simply not very well understand the CHECKSUM TABLE, but to me it doesn't seem useful to provide data integrity in the sense we're talking here about (i.e. silent block errors, bus errors, etc.) Why?
First, it seems to checksum the whole data of the whole table, and AFAICS, it uses only CRC32... given that such tables may be easily GiB in size, CRC32 is IMHO simply not good enough. Postgresql/btrfs in turn do the checksums on much smaller amounts of data.
Second, verification seems to only take place when that command is called. I'm not sure whether it implies locking the table in memory then (didn't dig too deep), but I can't believe it would, which system could keep a 100 GiB table in mem?
So it seems to be basically a one-shot verification, not covering any corruptions that happen in between.
In fact, the documentation of the function even tells that this is for backups/rollbacks/etc. only... so it's absolutely not that kind of data
integrity protection we're talking about (and even for that purpose,
CRC32 seems to be a poor choice).

Quoted 3 lines

VDI is still widely used, because it's the default for Virtual Box when
creating a VM.

Guess I just disbelieved that VirtualBox is still widely used O;-)

Quoted 11 lines

VHD is way more widely used than it should be, solely
because there are insane people out there using Windows as a virtualization host. You also forgot VMDK, which is what VMWare uses almost exclusively, but I don't think it has built-in checksumming.

As for Xen, the BCP are to avoid using image files like the plague, and
use disks directly instead (or more commonly, use either LVM, or ZFS with zvols).

Anyway... what it comes down to: None of the VM image formats seem to support checksumming.

Quoted 10 lines

Quoted 7 lines

So given all that, the picture looks a bit different again, I think.
None of major FLOSS DBs doesn't do any checksumming per default, MySQL
doesn't seem to support it, AFAICT. No VM image format seems to even
support it.

Again, most of my intent in referring to those was that the application
or the Guest OS would do the verification itself.

I've answered that above already, IIRC (our mails get too lengthy O:-) ).
The guest OS doesn't verify more than what our typical host OS (Linux) does.
And that (except when btrfs with CoWed data is used ;-) does filesystem integrity verification - which is however not data integrity verification.

btw: That makes me think about something interesting:
If btrfs will ever support checksumming on no-CoWed data... then the documentation should describe, that depending on the actual scenario, it may make sense that btrfs filesystems inside the guest run generally with nodatasum.
The idea begin: Why verifying twice?

The constraints being, AFAICS, the following:
- If the VM image is ever to be moved off the host's btrfs image (which
would have checksumming enabled) to a fs without checksumming or if it would be ever copied remotely, than not having the "internal" checksumming (i.e. from the btrfs filesystems inside the gues),
would
make one loose the integrity protection
- It further would only work, if the hypervisor itself, would properly pass on any IO errors when it reads from the image files in the host's btrfs, to block IO errors for the guest. If it wouldn't, then the guest (with disabled "internal" checksumming) wouldn't probably notice any data integrity errors, which he would, if the "internal" checksumming wasn't turned off.

Quoted 5 lines

If the application doesn't have that type of thing built in, then that's
not something the filesystem should be worrying about, that's the job of
the application developers to deal with.

No.

If you see it like that, you could as well drop data checksumming in btrfs completely.
You'd anyway argue that it would be the applications duty to do that.

The same way you could argue, that your MP4, JPG, ISO image or whatever you downloaded via bittorrent needs to contain checksum data, and that the actual application (which is not bittorrent, but e.g. mplayer, imagemagick or wodim) need to verify these.
However this is not the case.

In fact, it's quite usual and proper to have the lower layer handle stuff which the higher layer don't have any real direct interest in (except for the case, that the lower layer doesn't do it).

Quoted 6 lines

The point of a filesystem is
to store data within the integrity guarantees provided by the hardware,
possibly with some additional protection

If you're convinced by that, you should probably propose that btrfs removes data checksumming altogether.
I guess you won't make much friends with that idea ;)

Quoted 6 lines

In the case of stuff like torrents and such, all the good software for
working with them has an option to verify the file after downloading.

Not sure what you mean with "and such", if it's again VMs, and DBs (the IMHO actually more important use case than file sharing), I showed you in the last mail, that non of these do any verification by default, and
half of the important ones don't even support it, with nothing on the
horizon that this would change (probably because they argue just the other way round than you do: the fs should handle data integrity, and ZFS and btrfs give them partially right ;-) ).
And I'm not sure with torrents, but I'd have suspected once the file's downloaded completely, any checksumming data is no longer kept.

If my guess is correct, even the torrent software doesn't really do overall data integrity protection, but just until the download is finished; at least this used to be the case with the other P2P network softwares.

Thanks for the discussion so far :)
It actually made me just more confident that no-CoWed data checksumming should work and is actually needed ;)

Quoted 5 lines · Austin S. Hemmelgarn

Quoted 3 lines

nodatacow only [avoids fragmentation] if the file is pre-allocated, if it isn't, then it still ends up fragmented.

Quoted 3 lines

Hmm is that "it may end up fragmented" or a "it will definitely? Cause I'd have hoped, that if nothing else had been written in the meantime, btrfs would perhaps try to write next to the already allocated blocks.

If there are multiple files being written, then there is a relatively high probability that they will end up fragmented if they are more than about 64k and aren't pre-allocated.

Austin S. Hemmelgarn posted on Mon, 21 Dec 2015 08:36:02 -0500 as excerpted:

Quoted 7 lines · Christoph Anton Mitterer

Quoted 5 lines · Austin S. Hemmelgarn

Quoted 3 lines

nodatacow only [avoids fragmentation] if the file is pre-allocated, if it isn't, then it still ends up fragmented.

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Hmm is that "it may end up fragmented" or a "it will definitely? Cause I'd have hoped, that if nothing else had been written in the meantime, btrfs would perhaps try to write next to the already allocated blocks.

If there are multiple files being written, then there is a relatively high probability that they will end up fragmented if they are more than about 64k and aren't pre-allocated.

Does the 30-second-by-default commit window (and similarly 30-second- default dirty-flush-time at the VFS level) modify this at all? It has been my assumption that same-file writes accumulated during this time should merge, increasing efficiency and decreasing fragmentation (both with and without nocow), tho of course further writes outside this 30- second window will likely trigger it, if other files have been written in parallel or in the mean time.