Re: [Qemu-devel] [PATCH 5/9] mirror: improve performance of mirroring of empty disk

[Eric Blake <eblake@redhat.com>]

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On 06/16/2016 04:10 AM, Stefan Hajnoczi wrote:

> 
> io_sectors currently only accounts for bytes written, not bytes read.
> 
> Therefore, I think we need:
> 
> /* Don't charge for efficient zero writes */
> if (drv->bdrv_co_pwrite_zeroes) {
>     io_sectors = 0;
> }

That's not sufficient.  NBD will have conditional support for write
zeroes, depending on whether the server supports it (that is, once my
patches for NBD_CMD_WRITE_ZEROES get out of a holding pattern on all the
other patches in the queue being flushed...).  So NBD will have the
bdrv_co_pwrite_zeroes callback, but that doesn't mean it will always
work - if the server doesn't support it, calling bdrv_co_pwrite_zeroes
will fail with -ENOTSUP and fall back to less-efficient writes that need
to be accounted for.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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