Re: fstrim on BTRFS

[Chris Mason <chris.mason@oracle.com>]

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