Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH

[Paul Brook]

> Does this mean that virtio-blk supports all three combinations?
> 
>    1. FLUSH that isn't a barrier
>    2. FLUSH that is also a barrier
>    3. Barrier that is not a flush
> 
> 1 is good for fsync-like operations;
> 2 is good for journalling-like ordered operations.
> 3 sounds like it doesn't mean a lot as the host cache provides no
> guarantees and has no ordering facility that can be used.

(3) allows the guest to queue overlapping transfers with well defined results.
I have no idea how useful this is in practice, but it's certainly plausible.

Paul