Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog

[Anthony Liguori]

On 10/21/2010 09:07 AM, Kevin Wolf wrote:
> Hi all,
>
> I'm currently looking into adding a return value to qemu's bdrv_flush
> function and I noticed that your block drivers (nbd, rbd and sheepdog)
> don't implement bdrv_flush at all. bdrv_flush is going to return
> -ENOTSUP for any block driver not implementing this, effectively
> breaking these three drivers for anything but cache=unsafe.
>
> Is there a specific reason why your drivers don't implement this?

NBD doesn't have a notion of flush.  Only read/write and the block-nbd 
implementation doesn't do write-caching so flush would be a nop.

I'm not sure what the right semantics would be for QEMU.  My guess is a 
nop flush.

Regards,

Anthony Liguori

>   I
> think I remember that one of the drivers always provides
> cache=writethough semantics. It would be okay to silently "upgrade" to
> cache=writethrough, so in this case I'd just need to add an empty
> bdrv_flush implementation.
>
> Otherwise, we really cannot allow any option except cache=unsafe because
> that's the semantics provided by the driver.
>
> In any case, I think it would be a good idea to implement a real
> bdrv_flush function to allow the write-back cache modes cache=off and
> cache=writeback in order to improve performance over writethrough.
>
> Is this possible with your protocols, or can the protocol be changed to
> consider this? Any hints on how to proceed?
>
> Kevin
>
>