linux-ext4 - How to force EXT4_MB_GRP_CLEAR

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Date:   Sat, 23 Oct 2021 12:24:40 +0200
From:   Laurent GUERBY <laurent@...rby.net>
To:     linux-ext4@...r.kernel.org
Subject: How to force EXT4_MB_GRP_CLEAR_TRIMMED on a live ext4?

Hi,

When using fstrim on an ext4 filesystem trim are not issued for
EXT4_MB_GRP_WAS_TRIMMED space which is a useful optimization.

Is there a way to force a complete trim on a mounted ext4 filesystem? 

My (limited) understanding of the code is that
EXT4_MB_GRP_CLEAR_TRIMMED should be called to do so.

My use case is having live migrated a virtual machine root disk from
one storage to another, the target supporting trim, but since fstrim in
the VM post migration does mostly nothing (assumes most space was
trimmed) I cannot release space to the new storage.

I tried mount -o remount but without effect. e2fsprogs don't seem to
have an option/tool to do this either.

I've seen suggestion that rebooting will do the job but the whole point
of live migration is to avoid reboot :).

I did end up creating dummy files to fill the filesystem and then
removing them, but this is far less efficient than what a filesystem
tool could do.

Thanks in advance for your help,

Sincerely,

Laurent