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/
Please provide feedback.
Regards,
Raghavendra
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