Re: [PATCH] btrfs-progs: scrub: warn if scrub started on a device has mq-deadline

[David Sterba <dsterba@suse.cz>]

On Mon, Dec 07, 2020 at 07:23:03AM +0000, Johannes Thumshirn wrote:
> On 05/12/2020 19:51, Sidong Yang wrote:
> > Warn if scurb stared on a device that has mq-deadline as io-scheduler
> > and point documentation. mq-deadline doesn't work with ionice value and
> > it results performance loss. This warning helps users figure out the
> > situation. This patch implements the function that gets io-scheduler
> > from sysfs and check when scrub stars with the function.
> 
> From a quick grep it seems to me that only bfq is supporting ioprio settings.

Yeah it's only BFQ.

> Also there's some features like write ordering guarantees that currently 
> only mq-deadline provides.
> 
> This warning will trigger a lot once the zoned patchset for btrfs is merged,
> as for example SMR drives need this ordering guarantees and therefore select
> mq-deadline (via the ELEVATOR_F_ZBD_SEQ_WRITE elevator feature).

This won't affect the default case and for zoned fs we can't simply use
BFQ and thus the ionice interface. Which should be IMHO acceptable.