Re: How to force EXT4_MB_GRP_CLEAR_TRIMMED on a live ext4?

On Oct 25, 2021, at 09:29, Laurent GUERBY [off-list ref] wrote:

On Mon, 2021-10-25 at 11:42 +0200, Lukas Czerner wrote:

quoted

quoted

On Sat, Oct 23, 2021 at 12:24:40PM +0200, Laurent GUERBY wrote:

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.

Yeah, that's bad. The information is stored in the buddy cache in
memory
and AFAIK is only dropped on unmount. I'll have to think about how to
clear either the cache or selectively just the flag.

What would be more convenient way of doing this for you, -o remount,
or
using let's say tune2fs ? I am not promising anything yet, but I'll
think
about how to implement it.


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.

Thanks for the advice on fallocate! It does work and is very fast.

It would be enough to allocate and free a block in each group (128MB)
of the filesystem. That can't be controlled directly by fallocate(),
but indirectly via the "goal inode", but if fallocate() if all free space is fast
enough it may not be worth the effort. 

I would prefer a specific tune2fs as remount forcing this TRIM cache
clearing behaviour might be unwanted.

quoted

You could also force fsck on ro file system and use -E discard to
trim the
free space but I can't say I recommend it.

This would have a danger to corrupt the mounted filesystem, so should
not be allowed when doing a read-only e2fsck. The bitmaps that e2fsck
is using for the trim may be stale if the filesystem is modified since the
start of the run, which may be a long time in some cases (minutes, hours).

Cheers, Andreas