On 3/22/19 9:21 AM, Eric Blake wrote: > On 3/22/19 8:21 AM, Kevin Wolf wrote: >> If qemu-img convert sees that the target image isn't zero-initialised >> yet, it tries to do an efficient zero write for the whole image first >> to save the overhead of repeated explicit zero writes during the >> conversion. Obviously, this provides only an advantage if the >> pre-zeroing is actually efficient. Otherwise, we can end up writing >> zeroes slowly while zeroing out the whole image, and then overwrite the >> same blocks again with real data, potentially doubling the written data. >> >> Additionally, commit 9776f0db changed NBD to advertise for all NBD >> devices that zero writes with unmap is supported for all NBD devices, no >> matter whether the storage of NBD server actually can do this or whether >> we would fall back to explicit zero writes. >> >> This series adds a new BDRV_REQ_NO_FALLBACK flag for writing zeroes, >> which makes the request return an error if it can't guarantee that we >> don't end up running a slow fallback path. >> >> For NBD, this means that we still support zero writes from guests, but >> qemu-img convert will not try to use it to zero out the whole image. >> This is potentially suboptimal if the server does actually support >> efficient zero writes. We'd need an NBD protocol extension to transfer >> this flag so we can re-enable the qemu-img convert feature for the NBD >> driver without regressing on storage that can't provide efficient zero >> writes. > > I haven't reviewed closely yet, but the summary is sane, so: > Acked-by: Eric Blake <ebl...@redhat.com> > > and I'll be cross-posting another mail soon with the desired NBD > protocol extension.
I also meant to add - since the performance slowdown is so noticeable, and we are still at -rc0, I think this series is reasonable as 4.0 material as a bug fix (performance bugs rank lower than correctness bugs, but they are still bugs) -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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