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

[Kevin Wolf <kwolf@redhat.com>]

Am 08.01.2013 11:39, schrieb Liu Yuan:
> On 01/08/2013 06:00 PM, Kevin Wolf wrote:
>> Am 08.01.2013 10:45, schrieb Liu Yuan:
>>> On 01/08/2013 05:40 PM, Stefan Hajnoczi wrote:
>>>> Otherwise use sheepdog writeback and let QEMU block.c decide when to
>>>> flush.  Never use sheepdog writethrough because it's redundant here.
>>>
>>> I don't get it. What do you mean by 'redundant'? If we use virtio &
>>> sheepdog block driver, how can we specify writethrough mode for Sheepdog
>>> cache? Here 'writethrough' means use a pure read cache, which doesn't
>>> need flush at all.
>>
>> A writethrough cache is equivalent to a write-back cache where each
>> write is followed by a flush. qemu makes sure to send these flushes, so
>> there is no need use Sheepdog's writethrough mode.
> 
> Implement writethrough as writeback + flush will cause considerable
> overhead for network block device like Sheepdog: a single write request
> will be executed as two requests: write + flush

Yeah, maybe we should have some kind of a FUA flag with write requests
instead of sending a separate flush.

> This also explains why
> I saw a regression about write performance: Old QEMU can issue multiple
> write requests in one go, but now the requests are sent one by one (even
> with cache=writeback set), which makes Sheepdog write performance drop a
> lot. Is it possible to issue multiple requests in one go as old QEMU does?

Huh? We didn't change anything to that respect, or at least not that I'm
aware of. qemu always only had single-request bdrv_co_writev, so if
anything that batching must have happened inside Sheepdog code? Do you
know what makes it not batch requests any more?

> It seems it is hard to restore into old semantics of cache flags due to
> new design of QEMU block layer. So will you accept that adding a 'flags'
> into BlockDriverState which carry the 'cache flags' from user to keep
> backward compatibility?

No, going back to the old behaviour would break guest-toggled WCE.

Kevin