On Tue, Aug 17, 2010 at 10:17:15AM +0200, Tejun Heo wrote:

quoted

quoted

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.