RE: [Qemu-devel][PATCH,RFC] Zero cluster dedup

[Laurent Vivier <Laurent.Vivier@bull.net>]

Le mercredi 03 septembre 2008 à 00:35 -0700, Shahar Frank a écrit :
> > -----Original Message-----
> > From: Laurent Vivier [mailto:Laurent.Vivier@bull.net]
> > Sent: Wednesday, September 03, 2008 10:10 AM
> > To: qemu-devel@nongnu.org
> > Cc: Shahar Frank
> > Subject: Re: [Qemu-devel][PATCH,RFC] Zero cluster dedup
> > 
> > 
> > Is it really needed to have a shared zero page ?
> > 
> > When I read qcow_read() and qcow_aio_read_cb() I see:
> > 
> > if (!cluster_offset) {
> > ...
> > memset(buf, 0, 512 * n);
> > ...
> > }
> > and so I think you have just to clear the l2_table entry for the
> given
> > clusters (and free them).
> 
> You are correct if this is a standalone image i.e. without backing
> file. If the image has a backing file, a zero mapping is interpreted
> as "read it from the backing file at the same (logical) offset". 

I think backing file is also qcow2, so if you write all zeros the
behavior will be the same, and read from the backing file will use also
memset().

Did you try ?

Regards,
Laurent
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