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

[Laurent Vivier]

Le jeudi 21 octobre 2010 à 10:07 -0500, Anthony Liguori a écrit :
> 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.

I agree.

Regards,
Laurent

> 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
> >
> >    
> 

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