Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics

[Liu Yuan <namei.unix@gmail.com>]

On 01/07/2013 09:23 PM, Kevin Wolf wrote:
> No, and in theory they shouldn't have to care.
> 
> Why do you want to handle writethrough semantics in the block driver
> rather than letting qemu care for sending the right flushes?

Because Sheepdog itself provide the client side cache implementation and
we need means to get the cache hint from users when starting up the VM.
This cache support:
 writethrough: provide a read cache for this VM, which is always
consistent with cluster data
 writeback: provide a writeback cache for this VM, which only flush the
dirty bits when VM send 'flush' request.
 directio: disable cache completely for this VM.

Old QEMU (before v1.2) allows block drivers to get the cache flags, so
Sheepdog can interpret these flags on its own to choose the right cache
mode for each VM. doesn't QEMU need keep backward compatibility?

Thanks,
Yuan