Re: [Qemu-devel] [PATCH 1/2] qemu-img: Special post-backing convert handling

[Eric Blake]

On 05/01/2018 11:57 AM, Max Reitz wrote:
> Currently, qemu-img convert writes zeroes when it reads zeroes.
> Sometimes it does not because the target is initialized to zeroes
> anyway, so we do not need to overwrite (and thus potentially allocate)
> it.  This is never the case for targets with backing files, though.  But
> even them may have an area that is initialized to zeroes, and that is

s/them/they/

> the area past the end of the backing file (if that is shorter than the
> overlay).
>
> So if the target format's unallocated blocks are zero and there is a gap
> between the target's backing file's end and the target's end, we do not
> have to explicitly write zeroes there.
>
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898
> Signed-off-by: Max Reitz <address@hidden>
> ---
>   qemu-img.c | 26 +++++++++++++++++++++++++-
>   1 file changed, 25 insertions(+), 1 deletion(-)

Some long variable names, but it aids legibility.  Nice fix.

Reviewed-by: Eric Blake <address@hidden>

(And here's hoping that Red Hat someday flips the switch so that new 
images avoid compat=0.10 in their downstream builds, as we've had quite 
a few corner cases where we already have optimized speed for compat=1.1 ...)

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org