Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request toBIO

[Paolo Bonzini]

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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