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

[Jamie Lokier <jamie@shareable.org>]

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.

> 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.

-- Jamie