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

[Kevin Wolf <kwolf@redhat.com>]

Am 22.10.2010 07:43, schrieb MORITA Kazutaka:
> 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.

It could probably be optimized so that you only send flush requests to
servers that have actually received write requests since the last flush.

But yes, that's probably a valid point. I guess there's only one way to
find out how it performs: Trying it out.

> 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.

I agree, that makes sense.

Note that O_DIRECT does not provide write-through semantics. It bypasses
the page cache, but it doesn't flush other caches like a volatile disk
write cache. If you want to use it, you still need explicit flushes or
O_DIRECT | O_SYNC.

Kevin