[pve-devel] [PATCH docs] qm: guest trim: add note mentioning issue with ext4

[Fiona Ebner]

Am 10.03.23 um 10:43 schrieb Laurent GUERBY:
> On Fri, 2023-03-10 at 10:06 +0100, Fiona Ebner wrote:
>> It is rather unexpected and seems worth mentioning. Reported in the
>> community forum [0] and the explanation found by Alwin [1].
>>
>> [0]: https://forum.proxmox.com/threads/123819/
>> [1]: https://serverfault.com/questions/1113127/fstrim-is-very-slow-
>> on-xfs-and-always-return-same-value-unlike-ext4/1113129#1113129
> 
> Hi,
> 
> Here a workaround proposed by kernel developper when I reported the
> issue:
> 
> https://lore.kernel.org/all/1634984680.26818.10.camel@guerby.net/
> "How to force EXT4_MB_GRP_CLEAR_TRIMMED on a live ext4?"
> 
> "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."
> (...)
> "
> Meanwhile other than umount/mount, or actually writing to the dummy
> files,
> you can try to use fallocate to allocate all the remaining space in the
> file system and subsequently removing it. That should be more
> efficient,
> but don't forget to sync after remove to make sure the space is
> released
> before you call fstrim."
> 

Hi,
while we could mention that workaround, I'm not sure it's not too hacky
and filling up (almost) all space is not something I'd like to do in
production even if it's just very briefly ;)

> 
> I haven't followed up to see if code was added to deal with this
> (tune2fs?) without using fallocate.
> 

Remounting my root partition (even with the 'discard' mount option) in a
Debian 11 VM didn't help at least and I don't see anything other than
the 'discard' mount option in 'man tune2fs' either (searched for 'trim'
and 'discard').

Best Regards,
Fiona