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


From: MORITA Kazutaka
Subject: Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog
Date: Fri, 22 Oct 2010 14:43:46 +0900
User-agent: Wanderlust/2.14.0 (Africa) SEMI/1.14.6 (Maruoka) FLIM/1.14.9 (Gojō) APEL/10.8 Emacs/22.3 (x86_64-pc-linux-gnu) MULE/5.0 (SAKAKI)

At Thu, 21 Oct 2010 16:07:28 +0200,
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? 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?
> 

It is a bit difficult to implement an effective bdrv_flush in the
sheepdog block driver.  Sheepdog virtual disks are splited and
distributed to all cluster servers, so the block driver needs to send
flush requests to all of them.  I'm not sure this could improve
performance more than writethrough semantics.

So I think it is better to support only writethrough semantics
currently (I'll modify sheepdog server codes to open stored objects
with O_SYNC or O_DIRECT) and leave write-back semantics as a future
work.

Thanks,

Kazutaka



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