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SubjectRe: XFS mounted with 'discard' option - deleting fio test files slow
This is expected.  To get a better performance of XFS on NVMe drive, it
is advised not use use the realtime discard option.

https://www.intel.com/content/dam/support/us/en/documents/ssdc/data-center-ssds/Intel_Linux_NVMe_Guide_330602-002.pdf

Hao

==== Lu, Qian <luqia@amazon.com> [2017-09-08 17:17:07 +0000] ====

> Adding amazon-linux-kernel@amazon.com
>
> On 9/7/17, 11:22 AM, "Lu, Qian" <luqia@amazon.com> wrote:
>
> 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@ip-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
>
>
>
>

--
Hao Chen
Cordite SDM@Cupertino
408-480-6632

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