[Sheepdog] Reclaim free space using ATA discard

[Wido den Hollander]

Hi,

I found one post at the qemu-devel list regarding this feature, see:

http://www.archivum.info/qemu-devel@nongnu.org/2009-03/00220/%
28Qemu-devel%
29-Using-the-ATA-quot-trim-quot-command-to-discard-sectors-in-disk-images.html

I couldn't find any more posts then this one, so no idea when it will be
implemented in Qemu.

-- 
Met vriendelijke groet,

Wido den Hollander
Hoofd Systeembeheer / CSO
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On Tue, 2010-04-13 at 02:27 +0000, FUJITA Tomonori wrote:
> On Mon, 12 Apr 2010 12:46:49 +0200
> Wido den Hollander <wido at pcextreme.nl> wrote:
> 
> > Since kernel version 2.6.33 Linux supports the "discard" commando,
> > mainly used by SSD's for their wear-leveling and performance.
> > 
> > It's also called TRIM: http://en.wikipedia.org/wiki/TRIM
> > 
> > At the moment only Btrfs and EXT4 support trim/discard, but this could
> > be a nice feature for sheepdog.
> > 
> > If sheepdog reads the "discard" messages from the kernel in the VM you
> > could figure out which blocks are free again and shrink the used space
> > automatically.
> 
> Yeah, I saw Christoph's demonstration that the guest kernel issues a
> trim command then qemu's image shrinks on the host. We could do
> similar.
> 
> The version of qemu that we use now for Sheepdog doesn't notify the
> block drivers when the host kernel issues a trim command (or SCSI
> equivalents). I'm not sure about the latest version. But when qemu is
> ready, then we would work on sheepdog about this feature.