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

[Laurent Vivier]

Le mercredi 03 septembre 2008 à 10:13 +0200, Kevin Wolf a écrit :
> Laurent Vivier schrieb:
> > Le mercredi 03 septembre 2008 à 00:35 -0700, Shahar Frank a écrit :
> >>> 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().
> 
> Not if the cluster is already used in the backing file. Then you would
> write 0 to the L2 table of the image itself but not to the L2 table of
> the backing file (if you did, it would be a bug). So a read would
> operate on the old allocation of the backing file rather than on zeros.

Yes, you're right.

Laurent
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