On Thu, Jul 02, 2015 at 09:57:07AM +0200, Paolo Bonzini wrote:
> VIRTIO_BLK_F_CONFIG_WCE is important in order to achieve good performance
> (up to 2x, though more realistically +30-40%) in latency-bound workloads.
> However, it was removed by mistake together with VIRTIO_BLK_F_FLUSH.
>
> In addition, even removing VIRTIO_BLK_F_FLUSH was probably not a great
> idea, because it simplifies simple drivers (e.g. firmware) that are okay
> with a writethrough cache but still need data to persist after power loss.
> What really should have been removed is just the possibility that devices
> not propose VIRTIO_BLK_F_FLUSH, but even that only deserves a "SHOULD" in
> the new world of conformance statements.
>
> Restore these, with the following changes:
>
> * clarify and use conformance statements in order to define writeback
> and writethrough caching according to what is commonly done by high-end
> storage.
>
> * clarify (with conformance statements) the influence of the
> VIRTIO_BLK_F_FLUSH feature on caching and how to proceed if only one of
> VIRTIO_BLK_F_FLUSH and VIRTIO_BLK_F_CONFIG_WCE is negotiated.
>
> * strengthen the requirement for persisting writes after a
> VIRTIO_BLK_T_FLUSH request. This is now a MUST.
>
> This fixes VIRTIO-144.
>
> Signed-off-by: Paolo Bonzini <>
> ---
> content.tex | 76 ++++++++++++++++++++++++++++++++++++++++++-------------------
> 1 file changed, 52 insertions(+), 24 deletions(-)
Reviewed-by: Stefan Hajnoczi <>
