Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Avi Kivity]

 On 09/10/2010 02:33 PM, 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

Looks like it depends on fsck, which is not a good idea for large images.

> 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.

To deal with a sub-cluster TRIM, look at the surrounding sectors.  If 
they're zero, free the cluster.  If not, write zeros or use sys_punch() 
to the range specified by TRIM.


--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.