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SubjectRe: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO
On 01/02/2012 05:15 PM, Christoph Hellwig wrote:
> > When QEMU uses O_DIRECT, the guest should not use QUEUE_FLAG_NONROT
> > unless it is active for the host disk as well. (In doubt, as is the
> > case for remote hosts accessed over NFS, I would also avoid NONROT
> > and allow more coalescing).
>
> Do we have any benchmark numbers where QUEUE_FLAG_NONROT makes a
> difference?

Not that I know of.

> I tried a few times, and the only constant measureable
> thing was that it regressed performance when used for rotating devices
> in a few benchmarks.

Were you trying with cache=none or writeback? For cache=none, that's
exactly what I'd expect. cache=writeback could be more interesting...

Paolo


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