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

[Kevin Wolf <kwolf@redhat.com>]

Am 08.01.2013 12:08, schrieb Liu Yuan:
> On 01/08/2013 06:51 PM, Kevin Wolf wrote:
>> Am 08.01.2013 11:39, schrieb Liu Yuan:
>>> 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?
>>
> 
> QEMU v1.1.x works well with batched write requests. Sheepdog block
> driver doesn't do batching trick as far as I know, just send request as
> it is feed. There isn't noticeable changes between v1.1.x and current
> master regard to Sheepdog.c.
> 
> To detail the different behavior, from Sheepdog daemon which receives
> the requests from QEMU:
>  old: can receive multiple many requests at the virtually the same time
> and handle them concurrently
>  now: only receive one request, handle it, reply and get another.
> 
> So I think the problem is, current QEMU will wait for write response
> before sending another request.

I can't see a reason why it would do that. Can you bisect this?

>>> 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.
>>
> 
> Guest-toggled WCE only works with IDE and seems that virtio-blk doesn't
> support it, no? And I think there are huge virtio-blk users.

It works with virtio-blk and SCSI as well.

> I didn't meant to break WCE. What I meant is to allow backward
> compatibility. For e.g, Sheepdog driver can make use of this dedicated
> cache flags to implement its own cache control and doesn't affect other
> drivers at all.

How would you do it? With a WCE that changes during runtime the idea of
a flag that is passed to bdrv_open() and stays valid as long as the
BlockDriverState exists doesn't match reality any more.

Kevin