Re: fstrim on raid1 LV with writemostly PV leads to system freeze

[Roman Mamedov <rm@romanrm.net>]

On Thu, 21 Sep 2023 17:45:24 -0400
Mike Snitzer <snitzer@kernel.org> wrote:

> I just verified that 6.5.0 does have this DM core fix (needed to
> prevent excessive splitting of discard IO.. which could cause fstrim
> to take longer for a DM device), but again 6.5.0 has this fix so it
> isn't relevant:
> be04c14a1bd2 dm: use op specific max_sectors when splitting abnormal io
> 
> Given your use of 'writemostly' I'm inferring you're using lvm2's
> raid1 that uses MD raid1 code in terms of the dm-raid target.
> 
> Discards (more generic term for fstrim) are considered writes, so
> writemostly really shouldn't matter... but I know that there have been
> issues with MD's writemostly code (identified by others relatively
> recently).
> 
> All said: hopefully someone more MD oriented can review your report
> and help you further.
> 
> Mike  

I've reported that write-mostly TRIM gets split into 1MB pieces, which can be
an order of magnitude slower on some SSDs: https://www.spinics.net/lists/raid/msg72471.html

Nobody cared to reply, investigate or fix.

Maybe your system hasn't frozen too, just taking its time in processing all
the tiny split requests.

-- 
With respect,
Roman