Re: [Qemu-devel] [PATCH] block-raw: Make cache=off default again

[Kevin Wolf <kwolf@redhat.com>]

Jamie Lokier schrieb:
> Kevin Wolf wrote:
>> Jamie Lokier schrieb:
>>> Kevin Wolf wrote:
>>>> What happens with virtio I still need to understand. Obviously, as soon
>>>> as virtio decides to fall back to 4k requests, performance becomes
>>>> terrible.
>>> Does emulating a disk with 4k sector size instead of 512 bytes help this?
>> I just changed the virtio_blk code to always do the
>> blk_queue_hardsect_size with 4096, didn't change the behaviour.
> 
> You need quite a bit more than that to emulate a 4k sector size disk.
> There's the ATA/SCSI ID pages to update, and the special 512-bit
> offset tricky thing.

Okay, then I'll just admit that I know too little about the Linux block
layer. I'll gladly try any patch (and hopefully understand it then), but
for doing it myself I think it would take me a lot of time experimenting
and still not knowing if I'm doing it right.

>> I'm not sure if I have mentioned it in this thread: We have found that
>> it helps to use the deadline elevator instead of cfq in either the host
>> or the guest. I would accept this if it would only help when it's
>> changed in the guest (after all, I don't know the Linux block layer very
>> well), but I certainly don't understand how the host elevator could
>> change the guest request sizes - and noone else on the internal mailing
>> lists had an explanation either.
> 
> The host elevator will certainly affect the timing of I/O requests,
> which it receives from the guest, and it will also affect how requests
> are merged to make larger requests.
> 
> So it's not surprising that the host elevator changes the sizes of
> request sizes when they reach the host disk.
> 
> It shouldn't change the size of requests inside the guest, _before_
> they reach the host.

Yeah, this is exactly what I was thinking, too. However, in reality it
_does_ influence the guest request sizes, for whatever reason (maybe
again something timing related?). I put debug code into both the qemu
virtio-blk implementation and the guest kernel module and they both see
lots of 4k requests when the host uses cfg and much larger requests when
it uses deadline.

Kevin