linux-ext4 - discard and data=writeback

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Date:   Fri, 18 Dec 2020 19:40:09 +0100
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     linux-ext4@...r.kernel.org
Subject: discard and data=writeback

Hi,

I noticed a big slowdown on file removal, so I tried to remove the
discard option, and it helped
a lot.
Obviously discarding blocks will have an overhead, but the strange
thing is that it only
does when using data=writeback:

Ordered:

$ dmesg |grep EXT4
[    0.243372] EXT4-fs (vda1): mounted filesystem with ordered data
mode. Opts: (null)

$ grep -w / /proc/mounts
/dev/root / ext4 rw,noatime 0 0
$ time rm -rf linux-5.10

real    0m0.454s
user    0m0.029s
sys     0m0.409s

$ grep -w / /proc/mounts
/dev/root / ext4 rw,noatime,discard 0 0
$ time rm -rf linux-5.10

real    0m0.554s
user    0m0.051s
sys     0m0.403s

Writeback:

$ dmesg |grep EXT4
[    0.243909] EXT4-fs (vda1): mounted filesystem with writeback data
mode. Opts: (null)

$ grep -w / /proc/mounts
/dev/root / ext4 rw,noatime 0 0
$ time rm -rf linux-5.10

real    0m0.440s
user    0m0.030s
sys     0m0.407s

$ grep -w / /proc/mounts
/dev/root / ext4 rw,noatime,discard 0 0
$ time rm -rf linux-5.10

real    0m3.763s
user    0m0.030s
sys     0m0.876s

It seems that ext4_issue_discard() is called ~300 times with data=ordered
and ~50k times with data=writeback.
I'm using vanilla 5.10.1 kernel.

Any thoughts?

Regards,
-- 
per aspera ad upstream