QEMU has an optimization for a just-created drive-mirror destination
that is not possible for blockdev-mirror (which can't create the
destination) - any time we know the destination starts life as all
zeroes, we can skip a pre-zeroing pass on the destination. Recent
patches have added an improved heuristic for detecting if a file
contains all zeroes, and we plan to use that heuristic in upcoming
patches. But since a heuristic cannot quickly detect all scenarios,
and there may be cases where the caller is aware of information that
QEMU cannot learn quickly, it makes sense to have a way to tell QEMU
to assume facts about the destination that can make the mirror
operation faster. Given our existing example of "qemu-img convert
--target-is-zero", it is time to expose this override in QMP for
blockdev-mirror as well.
This patch results in some slight redundancy between the older
s->zero_target (set any time mode==FULL and the destination image was
not just created - ie. clear if drive-mirror is asking to skip the
pre-zero pass) and the newly-introduced s->target_is_zero (in addition
to the QMP override, it is set when drive-mirror creates the
destination image); this will be cleaned up in the next patch.
There is also a subtlety that we must consider. When drive-mirror is
passing target_is_zero on behalf of a just-created image, we know the
image is sparse (skipping the pre-zeroing keeps it that way), so it
doesn't matter whether the destination also has "discard":"unmap" and
"detect-zeroes":"unmap". But now that we are letting the user set the
knob for target-is-zero, if the user passes a pre-existing file that
is fully allocated, it is fine to leave the file fully allocated under
"detect-zeroes":"on", but if the file is open with
"detect-zeroes":"unmap", we should really be trying harder to punch
holes in the destination for every region of zeroes copied from the
source. The easiest way to do this is to still run the pre-zeroing
pass (turning the entire destination file sparse before populating
just the allocated portions of the source), even though that currently
results in double I/O to the portions of the file that are allocated.
A later patch will add further optimizations to reduce redundant
zeroing I/O during the mirror operation.
Since "target-is-zero":true is designed for optimizations, it is okay
to silently ignore the parameter rather than erroring if the user ever
sets the parameter in a scenario where the mirror job can't exploit it
(for example, when doing "sync":"top" instead of "sync":"full", we
can't pre-zero, so setting the parameter won't make a speed
difference).
Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250509204341.3553601-23-eblake@redhat.com>
Reviewed-by: Sunny Zhu <sunnyzhyy@qq.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
d17a34bfb9