Re: [PATCH 2/5] virtio_blk: implement REQ_FLUSH/FUA support

[Christoph Hellwig <hch@lst.de>]

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.