Re: fstrim silently does nothing on dev add/dev rem'd filesystem

[lutz.euler@freenet.de (Lutz Euler)]

Hi Chris,

> I have a filesystem for which fstrim won't do anything.
> The filesystem has a history of abuse; dev add, dev remove, dding, ...
> 
> There's nothing wrong with the kernel or the disc; other btrfs volumes
> on the same disc trim fine, and the volume used to trim fine.
> 
> By "won't trim", I mean that it always, instantly returns 0 bytes
> trimmed:

you probably suffer from the same problem I had a few years ago.
It is a bug in how btrfs implements fstrim. Adding and removing devices
is the way I got my btrfs filesystem into the same situation then.

See for the background:
http://comments.gmane.org/gmane.comp.file-systems.btrfs/15597

I wrote a patch, see here:
http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg40618.html

I understand that in kernel 4.3 there will be an extension so that
fstrim trims free space outside of allocated chunks. This is orthogonal;
my patch allows to trim free space inside allocated chunks (which,
somewhat accidentally, works for most btrfs filesystems since a long
time) even with filesystems like yours.

Kind regards,

Lutz Euler