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

[Stefan Hajnoczi]

On Thu, Jun 16, 2016 at 08:53:12PM -0600, Eric Blake wrote:
> 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.

Good point!  Optimizations are tricky :-).

Stefan

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