Re: [Qemu-devel] [PATCH] sheepdog: implement SD_OP_FLUSH_VDI operation

[MORITA Kazutaka]

At Fri, 20 Apr 2012 20:05:48 +0200,
Christoph Hellwig wrote:
> 
> On Tue, Apr 03, 2012 at 01:35:50AM +0800, Liu Yuan wrote:
> > From: Liu Yuan <address@hidden>
> > 
> > Flush operation is supposed to flush the write-back cache of
> > sheepdog cluster.
> > 
> > By issuing flush operation, we can assure the Guest of data
> > reaching the sheepdog cluster storage.
> 
> How does qemu know that the cache might need flushing in the backend?
> 
> If the backend expects a flush we need to make sure one is sent after
> every write if the user selects cache=writethrough or cache=directsync.
> Given how expensive that is you should also consider adding an equivalent
> to the FUA flag for writes in that case, or simply allow the qemu driver
> to tell the cluster that it needs to operate in writethrough mode for
> this VDI.

His patch sets the SD_FLAG_CMD_CACHE flag for writes only when the
user selects cache=writeback or cache=none.  If SD_FLAG_CMD_CACHE is
not set in the request, Sheepdog servers are forced to flush the cache
like FUA commands.

Thanks,

Kazutaka