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

[Christoph Hellwig]

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.