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

[Kevin Wolf <kwolf@redhat.com>]

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.

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.

Kevin