linux-kernel - XFS mounted with 'discard' option

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Date:   Thu, 7 Sep 2017 18:22:22 +0000
From:   "Lu, Qian" <luqia@...zon.com>
To:     "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: XFS mounted with 'discard' option - deleting fio test files slow

Hi XFS mailing list,

Recently we received a bug report in the XFS filesystem with 'discard' option. I have been able to reproduce this issue. I used XFS filesystem to format NVMe SSD and mounted with 'discard' option. When I tried to delete the test fio files, the session took long time. This issue is based on Linux 4.9 stable tree. I have also repeated this test with Linux 4.13, 4.12, and we are facing the same issue. Tests were repeated several times and it was consistent.

Please see details below.

1. Kernel version: Linux ip-172-31-6-243 4.9.32-15.41.amzn1.x86_64 #1 SMP Thu Jun 22 06:20:54 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

# fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
--> Interrupt with Ctrl+C
# time rm -rf fio_test_file.*
--> The session hangs and in 'blocked' state
$ dmesg
...
[  492.329896] INFO: task rm:9231 blocked for more than 120 seconds.
...

Then I tried to backport some patches and repeated the test. The issue has been improved. Eventually 'rm' command completed but took long time(2min).

  *  Backported patch: 4560e78 xfs: don't block the log commit handler for discards

 # fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
 --> Interrupt with Ctrl+C
 # time rm -rf fio_test_file.*
 real  2m2.242s
 user  0m0.000s
 sys   0m25.524s

2.    With Linux 4.12 and 4.13.0-rc1, the issue has been improved and the command is not stuck. But 'rm' command still takes long time (more than 1min). Please see details below.

Kernel version: Linux version 4.13.0-rc1+ (ec2-user@...172-31-21-25) (gcc version 4.8.3 20140911 (Red Hat 4.8.3-9) (GCC)) #1 SMP Fri Jul 21 17:31:06 UTC 2017

# fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
--> Interrupt at about 37%
# time rm -rf fio_test_file.*
real  1m57.912s
user  0m0.000s
sys   0m28.810s

Compare this result with:
    a)  XFS mounted with 'nodiscard' option: It took less than 1min to run 'rm' command.

     # fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
    --> Interrupt at about 39%
     # time rm -rf fio_test_file.*
    real     0m31.176s
     user     0m0.000s
     sys      0m30.005s

     b)    EXT4 file system mounted with 'discard' option: It only took about several seconds to run 'rm' command.

      # fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
     --> Interrupt at about 36.2%
     # time rm -rf fio_test_file.*
     real     0m4.661s
     user     0m0.000s
     sys      0m4.657s

Please note if I wait for 'fio' command 100% done, 'rm' command took less than 1s (0m0.001s).

3.   Shell script which triggers the problem

  sudo su -
  yum install xfsprogs fio -y
  mkfs.xfs -K -f -s size=4096 /dev/nvme0n1
  mkdir -p /media/disk1
  mount -o discard /dev/nvme0n1 /media/disk1
  cd /media/disk1/
  fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
   # Interrupt with Ctrl+C
  time rm -rf fio_test_file.*

Best Regards,
Qian Lu