Re: XFS Syncd

Dave Chinner
2015-06-04T01:55:29Z

On Wed, Jun 03, 2015 at 05:58:07PM -0700, Shrinand Javadekar wrote:
> Thanks Dave. Please see my responses inline.
>
> On Wed, Jun 3, 2015 at 5:35 PM, Dave Chinner <david@fromorbit.com> wrote:
> > On Wed, Jun 03, 2015 at 04:18:20PM -0700, Shrinand Javadekar wrote:
> >> Here you go!
> >
> > Thanks!
> >
> >> /dev/mapper/35000c50062e6a12b-part2 /srv/node/r1 xfs
> >> rw,nosuid,nodev,noexec,noatime,nodiratime,attr2,nobarrier,inode64,logbufs=8,noquota
> >> 0 0
> > .....
> >> meta-data=/dev/mapper/35000c50062e6a7eb-part2 isize=256 agcount=64, agsize=11446344 blks
> >> = sectsz=512 attr=2
> >> data = bsize=4096 blocks=732566016, imaxpct=5
> >> = sunit=0 swidth=0 blks
> >> naming =version 2 bsize=4096 ascii-ci=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
> >
> > Ok, so agcount=64 is unusual, especially for a single disk
> > filesystem. What was the reason for doing this?
>
> I read few articles that recommend using an increased number of AGs,
> especially when there are large disks. I can use the default # of AGs
> (4?) and try again.

<sigh>

The Google Fallacy strikes again.

http://xfs.org/index.php/XFS_FAQ#Q:_I_want_to_tune_my_XFS_filesystems_for_.3Csomething.3E

> >> Openstack Swift. This is what it's doing:
> >>
> >> 1. A path like /srv/node/r1/objects/1024/eef/tmp already exists.
> >> /srv/node/r1 is the mount point.
> >> 2. Creates a tmp file, say tmpfoo in the patch above. Path:
> >> /srv/node/r1/objects/1024/eef/tmp/tmpfoo.
> >> 3. Issues a 256KB write into this file.
> >> 4. Issues an fsync on the file.
> >> 5. Closes this file.
> >> 6. Creates another directory named "deadbeef" inside "eef" if it
> >> doesn't exist. Path /srv/node/r1/objects/1024/eef/deadbeef.
> >> 7. Moves file tmpfoo into the deadbeef directory using rename().
> >> /srv/node/r1/objects/1023/eef/tmp/tmpfoo -->
> >> /srv/node/r1/objects/1024/eef/deadbeef/foo.data
> >> 8. Does a readdir on /srv/node/r1/objects/1024/eef/deadbeef/
> >> 9. Iterates over all files obtained in #8 above. Usually #8 gives only one file.
> >
> > Oh. We've already discussed this problem in a previous thread:
> >
> > http://oss.sgi.com/archives/xfs/2015-04/msg00256.html
>
> Yes, we touched upon this earlier and found that all files were
> getting created in the same AG. We fixed that by and my current
> testing includes that fix.

Right, I noticed that looking at the inode allocation distribution.
It's pretty good (output is count, agno):

$$ awk '/xfs_ialloc_read_agi:/ {print $8}' trace_report.txt | sort -n |uniq -c
1362 0
1351 1
1359 2
1354 3
1374 4
1345 5
1380 6
1371 7
1356 8
1354 9
1373 10
1364 11
1357 12
1363 13
1368 14
1386 15
1355 16
1384 17
1352 18
1377 19
1358 20
1371 21
1356 22
1367 23
1342 24
1383 25
1352 26
1354 27
1347 28
1382 29
1348 30
1347 31
1351 32
1346 33
1350 34
1365 35
1346 36
1361 37
1358 38
1337 39
1356 40
1371 41
1347 42
1335 43
1378 44
1370 45
1372 46
1334 47
1363 48
1355 49
1365 50
1353 51
1370 52
1346 53
1369 54
1356 55
1381 56
1349 57
1365 58
1356 59
1351 60
1345 61
1379 62
1351 63

> > Specifically, that discussion touched on problems your workload
> > induces in metadata layout and locality:
> >
> > http://oss.sgi.com/archives/xfs/2015-04/msg00300.html
> >
> > And you are using agcount=64 on these machines, so that's going to
> > cause you all sorts of locality problems, which will translate into
> > seek bound IO performance....
> >
> >> - IOStat and vmstat output
> >> (attached)
> >
> > I am assuming these are 1 second samples, based on your 18s fast/12s
> > slow description earlier.
>
> Yes, these are 1 seconds samples.
>
> >
> > The vmstat shows fast writeback at 150-200MB/s, with no idle time,
> > anything up to 200 processes in running or blocked state and 20-30%
> > iowait, followed by idle CPU time with maybe 10 running/blocked
> > processes, writeback at 15-20MB/s with 70% idle time and 30% iowait.
> >
> > IOWs, the workload is cyclic - lots of incoming data with lots of
> > throughput, followed by zero incoming data processing on only small
> > amounts of writeback.
>
> My understanding is that the workload is either
>
> a) waiting for issued IOs to complete.
> b) not able to issue more IOs because XFS is busy flushing the journal entries.
>
> Is this not true?

It's just an *observation* that the incoming processing has stopped
from the data presented, and it doesn't speak to the cause of why
incoming data is not being processed. You're jumping to conclusions
again before there is supporting evidence to make such a statement.

> > The vmstat information implies that front end application processing
> > is stopping for some period of time, but it does not indicate why it
> > is doing so. When the disks are doing 4k writeback, can you grab
> > the output of 'echo w > /proc/sysrq-trigger' from dmesg and post the
> > output? That will tell us if the front end processing is blocked on
> > the filesystem at all...
>
> Aah.. ok. Will do and get back to you soon.

See? more information is required. ;)