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    SubjectRe: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO
    On Mon, Jan 02, 2012 at 05:12:00PM +0100, Paolo Bonzini wrote:
    > On 01/01/2012 05:45 PM, Stefan Hajnoczi wrote:
    > >By the way, drivers for solid-state devices can set QUEUE_FLAG_NONROT
    > >to hint that seek time optimizations may be sub-optimal. NBD and
    > >other virtual/pseudo device drivers set this flag. Should virtio-blk
    > >set it and how does it affect performance?
    > By itself is not a good idea in general.
    > 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? 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.

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