[Gluster-devel] fallocate behavior in glusterfs

Wed Jul 3 17:28:52 UTC 2019

On Wed, Jul 3, 2019 at 3:28 AM Pranith Kumar Karampuri <pkarampu at redhat.com>
wrote:

>
>
> On Wed, Jul 3, 2019 at 10:14 AM Ravishankar N <ravishankar at redhat.com>
> wrote:
>
>>
>> On 02/07/19 8:52 PM, FNU Raghavendra Manjunath wrote:
>>
>>
>> Hi All,
>>
>> In glusterfs, there is an issue regarding the fallocate behavior. In
>> short, if someone does fallocate from the mount point with some size that
>> is greater than the available size in the backend filesystem where the file
>> is present, then fallocate can fail with a subset of the required number of
>> blocks allocated and then failing in the backend filesystem with ENOSPC
>> error.
>>
>> The behavior of fallocate in itself is simlar to how it would have been
>> on a disk filesystem (atleast xfs where it was checked). i.e. allocates
>> subset of the required number of blocks and then fail with ENOSPC. And the
>> file in itself would show the number of blocks in stat to be whatever was
>> allocated as part of fallocate. Please refer [1] where the issue is
>> explained.
>>
>> Now, there is one small difference between how the behavior is between
>> glusterfs and xfs.
>> In xfs after fallocate fails, doing 'stat' on the file shows the number
>> of blocks that have been allocated. Whereas in glusterfs, the number of
>> blocks is shown as zero which makes tools like "du" show zero consumption.
>> This difference in behavior in glusterfs is because of libglusterfs on how
>> it handles sparse files etc for calculating number of blocks (mentioned in
>> [1])
>>
>> At this point I can think of 3 things on how to handle this.
>>
>> 1) Except for how many blocks are shown in the stat output for the file
>> from the mount point (on which fallocate was done), the remaining behavior
>> of attempting to allocate the requested size and failing when the
>> filesystem becomes full is similar to that of XFS.
>>
>> Hence, what is required is to come up with a solution on how libglusterfs
>> calculate blocks for sparse files etc (without breaking any of the existing
>> components and features). This makes the behavior similar to that of
>> backend filesystem. This might require its own time to fix libglusterfs
>> logic without impacting anything else.
>>
>> I think we should just revert the commit
>> b1a5fa55695f497952264e35a9c8eb2bbf1ec4c3 (BZ 817343) and see if it really
>> breaks anything (or check whatever it breaks is something that we can live
>> with). XFS speculative preallocation is not permanent and the extra space
>> is freed up eventually. It can be sped up via procfs tunable:
>> http://xfs.org/index.php/XFS_FAQ#Q:_How_can_I_speed_up_or_avoid_delayed_removal_of_speculative_preallocation.3F.
>> We could also tune the allocsize option to a low value like 4k so that
>> glusterfs quota is not affected.
>>
>> FWIW, ENOSPC is not the only fallocate problem in gluster because of
>> 'iatt->ia_block' tweaking. It also breaks the --keep-size option (i.e. the
>> FALLOC_FL_KEEP_SIZE flag in fallocate(2)) and reports incorrect du size.
>>
> Regards,
>> Ravi
>>
>>
>> OR
>>
>> 2) Once the fallocate fails in the backend filesystem, make posix xlator
>> in the brick truncate the file to the previous size of the file before
>> attempting fallocate. A patch [2] has been sent for this. But there is an
>> issue with this when there are parallel writes and fallocate operations
>> happening on the same file. It can lead to a data loss.
>>
>> a) statpre is obtained ===> before fallocate is attempted, get the stat
>> hence the size of the file b) A parrallel Write fop on the same file that
>> extends the file is successful c) Fallocate fails d) ftruncate truncates it
>> to size given by statpre (i.e. the previous stat and the size obtained in
>> step a)
>>
>> OR
>>
>> 3) Make posix check for available disk size before doing fallocate. i.e.
>> in fallocate once posix gets the number of bytes to be allocated for the
>> file from a particular offset, it checks whether so many bytes are
>> available or not in the disk. If not, fail the fallocate fop with ENOSPC
>> (without attempting it on the backend filesystem).
>>
>> There still is a probability of a parallel write happening while this
>> fallocate is happening and by the time falllocate system call is attempted
>> on the disk, the available space might have been less than what was
>> calculated before fallocate.
>> i.e. following things can happen
>>
>>  a) statfs ===> get the available space of the backend filesystem
>>  b) a parallel write succeeds and extends the file
>>  c) fallocate is attempted assuming there is sufficient space in the
>> backend
>>
>> While the above situation can arise, I think we are still fine. Because
>> fallocate is attempted from the offset received in the fop. So,
>> irrespective of whether write extended the file or not, the fallocate
>> itself will be attempted for so many bytes from the offset which we found
>> to be available by getting statfs information.
>>
>> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1724754#c3
>> [2] https://review.gluster.org/#/c/glusterfs/+/22969/
>>
>>
> option 2) will affect performance if we have to serialize all the data
> operations on the file.
> option 3) can still lead to the same problem we are trying to solve in a
> different way.
>          - thread-1: fallocate came with 1MB size, Statfs says there is
> 1MB space.
>          - thread-2: Write on a different file is attempted with 128KB and
> succeeds
>          - thread-1: fallocate fails on the file after partially
> allocating size because there doesn't exist 1MB anymore.
>
>
Here I have a doubt. Even if a 128K write on the file succeeds, IIUC
fallocate will try to reserve 1MB of space relative to the offset that was
received as part of the fallocate call which was found to be available.
So, despite write succeeding, the region fallocate aimed at was 1MB of
space from a particular offset. As long as that is available, can posix
still go ahead and perform the fallocate operation?

Regards,
Raghavendra

> So option-1 is what we need to explore and fix it so that the behavior is
> closer to other posix filesystems. Maybe start with what Ravi suggested?
>
>
>> Please provide feedback.
>>
>> Regards,
>> Raghavendra
>>
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>
> --
> Pranith
>
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