Re: fstrim on BTRFS

On Thu, Dec 29, 2011 at 12:02:48PM +0800, Li Zefan wrote:

Martin Steigerwald wrote:

quoted

Hi!

With 3.2-rc4 (probably earlier), Ext4 seems to remember what areas it 
trimmed:

merkaba:~> fstrim -v /boot
/boot: 224657408 bytes were trimmed
merkaba:~> fstrim -v /boot
/boot: 0 bytes were trimmed


But BTRFS does not:

merkaba:~> fstrim -v /
/: 4431613952 bytes were trimmed
merkaba:~> fstrim -v /    
/: 4341846016 bytes were trimmed


Is it planned to add this feature to BTRFS as well?

There's no such plan, but it's do-able, and I can take care of it.
There's an issue though.

Whether we want to store TRIMMED information on disk? ext4 doesn't
do this, so the first fstrim will be slow though you've done fstrim
in previous mount.

I'd rather not store the trim status on disk.  The extra trims
don't have a huge cost, and since some devices have a large granularity
for trims, they may ignore the trim until it tosses a larger contiguous
area of the disk.

I'd be fine with a flag to the in-memory free extent struct that
indicates if it has been trimmed down to the device.

-chris