Request for information on bloated writes using Swift

[Dilip Simha]

Hi Eric,

Thank you for your quick reply.

Using xfs_io as per your suggestion, I am able to reproduce the issue.
However, I need to falloc for 256K and write for 257K to see this issue.

# xfs_io -f -c "falloc 0 256k" -c "pwrite 0 257k" /srv/node/r1/t1.txt
# stat /srv/node/r1/t4.txt | grep Blocks
  Size: 263168     Blocks: 1536       IO Block: 4096   regular file

# xfs_io -f -c "pwrite 0 257k" /srv/node/r1/t2.txt
# stat  /srv/node/r1/t2.txt | grep Blocks
Size: 263168    *Blocks*: 520        IO Block: 4096   regular file

# xfs_info /srv/node/r1
meta-data=/dev/mapper/35000cca05831283c-part2 isize=256    agcount=4,
agsize=183141504 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=0        finobt=0
data     =                       bsize=4096   blocks=732566016, imaxpct=5
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal               bsize=4096   blocks=357698, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

# cat /proc/mounts | grep r1

/dev/mapper/35000cca05831283c-part2 /srv/node/*r1* xfs
rw,nosuid,nodev,noexec,noatime,nodiratime,attr2,inode64,logbufs=8,noquota 0
0
I waited for around 15 mins before collecting the stat output to give the
background reclamation logic a fair chance to do its job. I also tried
changing the value of speculative_prealloc_lifetime from 300 to 10. But it
was of no use.

cat /proc/sys/fs/xfs/speculative_prealloc_lifetime
10

Regards,
Dilip

On Tue, Feb 2, 2016 at 6:47 PM, Eric Sandeen <sandeen at sandeen.net> wrote:

>
>
> On 2/2/16 4:32 PM, Dilip Simha wrote:
> > Hi,
> >
> > I have a question regarding speculated preallocation in XFS, w.r.t
> > kernel version: 3.16.0-46-generic. I am using Swift version: 1.0 and
> > mkfs.xfs version 3.2.1
> >
> > When I write a 256KiB file to Swift, I see that the underlying XFS
> > uses 3x the amount of space/blocks to write that data. Upon
> > performing detailed experiments, I see that when Swift uses fallocate
> > (default approach), XFS doesn't reclaim the preallocated blocks that
> > XFS allocated. Swift fallocate doesn't exceed the body size(256
> > KiB).
> >
> > Interestingly, when either allocsize=4k or when swift doesn't use
> > fallocate, XFS doesn't consume additional space.
> >
> > Can you please let me know if this is a known bug and if its fixed in
> > the later versions?
>
> Can you clarify the exact sequence of events?
>
> i.e. -
>
> xfs_io -f -c "fallocate 0 256k" -c "pwrite 0 256k" somefile
>
> leads to unreaclaimed preallocation, while
>
> xfs_io -f -c "pwrite 0 256k" somefile
>
> does not?  Or is it some other sequence?  I don't have a
> 3.16 handy to test, but if you can describe it in more detail
> that'd help.  Some of this is influenced by fs geometry, too
> so xfs_info output would be good, along with any mount options
> you might be using.
>
> Are you preallocating with or without KEEP_SIZE?
>
> -Eric
>
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