[pve-devel] cache unsafe write for qemu-img convert, vma extract and vmstate snapshot

[Alexandre Derumier]

As reported by  Eneko Lacunza,
ceph/rbd by default use writethrough even if writeback is defined, until a flush is detected.

We currently use writeback without sendind any flush for 3 things:

qemu-img convert : (default is unsafe, but we force writeback in qemuserver)
vma restore on target destination
vmstate snapshot

The patches serie  set unsafe opening (BDRV_O_NO_FLUSH).

And also fix the rbd drive to disable cache_writethtrough_until_flush if cache=unsafe

@Eneko Lacunza : Can you test it ? I have rewrote your last patch (http://pve.proxmox.com/pipermail/pve-devel/2016-July/022226.html),
                to handle vma extract and vmstate snapshot