[Sheepdog] [QEMU PATCH] sheepdog: add cache flush support

[MORITA Kazutaka]

At Mon, 21 Nov 2011 05:19:26 -0500,
Christoph Hellwig wrote:
> 
> On Wed, Nov 16, 2011 at 09:58:14PM +0900, MORITA Kazutaka wrote:
> > > > If we increment SD_PROTO_VER, the sheep daemon should return
> > > > SD_RES_VER_MISMATCH error when opening the VDI.
> > > 
> > > And then revert to trying the old protocol revision if it fails?  Sounds
> > > like that would be worth a try.
> > 
> > Or perhaps it might be better not to support the old protocol in the
> > qemu block driver?  It will make the code complicated.
> 
> qemu has cache modes with and without a writeback cache, and the modes
> without it are absolutely required for supporting OSes that don't
> support cache flushing.  I would suggest we introduce a new protocol
> version, and that version will allow the server and client to negotiate
> optional feature bits.  The write cache would be the first one, but
> things like discard support would also be able to make use of it.

Sounds reasonable.

Thanks,

Kazutaka