Re: [PATCH v2] block: BFQ default for single queue devices

Bryan Gurney
2018-10-19T13:36:49Z

On Fri, Oct 19, 2018 at 4:42 AM, Linus Walleij <linus.walleij@linaro.org> wrote:
>
> On Wed, Oct 17, 2018 at 4:59 PM Bryan Gurney <bgurney@redhat.com> wrote:
>
> > I feel strongly about the prevention of users running into errors
> > because of an incorrect scheduler default, because I encountered that
> > situation three times in my testing with zoned block devices. The
> > switch to SCSI_MQ would resolve that, since mq-deadline is the
> > default, but in my case, I was using Fedora 28, which disables
> > CONFIG_SCSI_MQ_DEFAULT (which is enabled in the 4.18 kernel), so my
> > default scheduler was cfq.
>
> I think we should make a patch to the kernel that makes it
> impossible (even from sysfs) to choose a non-zone aware
> scheduler for these devices.
>
> It's another topic than $SUBJECT patch though. I take this
> into account in this version.
>

I like this idea. I don't have enough experience to write this patch
myself, but I imagine something like adding "bool is_zoned_aware" to
"struct elevator_type", and setting that true only for the schedulers
that are currently zoned-device aware (which is currently deadline on
single queue, mq-deadline on blk-mq).

Thanks,

Bryan