Re: [PATCH] virtio_blk: set the default scheduler to none

[Christoph Hellwig]

On Thu, Dec 07, 2023 at 03:55:17PM +0100, Paolo Bonzini wrote:
> > This seems similar to commit f8b12e513b95 ("virtio_blk: revert
> > QUEUE_FLAG_VIRT addition") where changing the default sounded good in
> > theory but exposed existing users to performance regressions. [...]
> > I don't want to be overly conservative. The virtio_blk driver has
> > undergone changes in this regard from the legacy block layer to blk-mq
> > (without an I/O scheduler) to blk-mq (mq-deadline).
> 
> IIRC there were also regressions in both virtio-blk and virtio-scsi
> when switching from the legacy block layer to blk-mq. So perhaps I
> *am* a bit more conservative, but based on past experience, this patch
> seems not to be a great idea for practical use cases.

Agreed.  I'm in fact not exactly happy about the rather odd
BLK_MQ_F_NO_SCHED_BY_DEFAULT flag to start with.