On Fri, Sep 10, 2010 at 12:33:09PM +0100, Stefan Hajnoczi wrote:
btw, despite being not properly designed, qcow2 is able to support TRIM.
?qed isn't able to, except by leaking clusters on shutdown. ?TRIM support is
required unless you're okay with the image growing until it is no longer
sparse (the lack of TRIM support in guests make sparse image formats
somewhat of a joke, but nobody seems to notice).
Anthony has started writing up notes on trim for qed:
http://wiki.qemu.org/Features/QED/Trim
I need to look at the actual ATA and SCSI specs for how this will
work. The issue I am concerned with is sub-cluster trim operations.
If the trim region is less than a cluster, then both qed and qcow2
don't really have a way to handle it. Perhaps we could punch a hole
in the file, given a userspace interface to do this, but that isn't
ideal because we're losing sparseness again.
ATA TRIM doesn't have a granularity, it's always sector sized. SCSI
WRITE SAME with the unmap bit or UNMAP as well as my virtio_blk support
for discarding blocks export topoligy information about the required
minimum discard request size. I export it from qemu the same way as
we export other topology information and at least Linux hosts can use
it.