Re: [Qemu-devel] [PATCH] Detect zero writes (for discussion only, not to be applied)

[Anthony Liguori <anthony@codemonkey.ws>]

"Richard W.M. Jones" <rjones@redhat.com> writes:

> [Applies against qemu 1.1.0, not HEAD, because qemu from git seems to
> be broken at the moment.  But the patch is easy to rebase.]

How's it broken?

Regards,

Anthony Liguori

> This patch adds a -drive detect_zero=on|off flag (defaults to "off").
>
> Setting this on causes the write path to detect all zero byte writes
> and, for block device drivers which support this, turn the request
> into the more space-efficient bdrv_co_do_write_zeroes call.
>
> The use case is for virt-sparsify.  We want to create an intermediate
> qcow2 overlay file which we know will mostly contain zeroes, and we
> want qcow2 to handle this without creating a huge empty file.  In my
> testing, the size of the intermediate file is reduced considerably
> (down to about 1/4 of the original size), making sparsification of
> large mostly-empty guests possible for the first time.
>
> The implementation is very simple-minded.  It tests if the entire
> request is all zero bytes or not.  A better implementation would
> almost certainly be even more space-efficient, but at the cost of
> splitting some write requests (if that's a problem).
>
> Testing: I booted and tested an F16 guest with this patch and the
> setting "on", without problems.  I also modified libguestfs to add
> this flag, then ran the whole libguestfs test suite with no reported
> problems.
>
> Rich.