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    SubjectRe: [PATCH 2/5] virtio_blk: implement REQ_FLUSH/FUA support
    On Tue, Aug 17, 2010 at 10:17:15AM +0200, Tejun Heo wrote:
    > >> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
    > >> support instead. A new feature flag VIRTIO_BLK_F_FUA is added to
    > >> indicate the support for FUA.
    > >
    > > I'm not sure it's worth it. The pure REQ_FLUSH path works not and is
    > > well tested with kvm/qemu. We can still easily add a FUA bit, and
    > > even a pre-flush bit if the protocol roundtrips matter in real life
    > > benchmarking.
    > Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
    > should be cheaper than FLUSH.

    If someone ever wrote a virtio-blk backend that sits directly ontop
    of the Linux block layer that would be true. Of the five known
    virtio-blk backends all operate on normal files using the Posix I/O
    APIs, or the Linux aio API (optionally in qemu) or in-kernel
    vfs_read/vfs_write (vhost-blk).

    Given how little testing lguest gets compared to qemu I really don't
    want a protocol addition for it unless it really buys us something.
    Once we're done with this barrier conversion I plan into benchmarking
    FUA and a pre-flush tag on the command for virtio in real life setups,
    and see if it actually buys us anything.

     \ /
      Last update: 2010-08-17 15:27    [W:0.020 / U:1.692 seconds]
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