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

[Paolo Valente]

> Il giorno 03 ott 2018, alle ore 10:28, Linus Walleij <linus.walleij@xxxxxxxxxx> ha scritto:
> 
> On Wed, Oct 3, 2018 at 9:42 AM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
> 
>> There is another class of outliers: host-managed SMR disks (SATA and SCSI,
>> definitely single hw queue). For these, using mq-deadline is mandatory in many
>> cases in order to guarantee sequential write command delivery to the device
>> driver. Having the default changed to bfq, which as far as I know is not SMR
>> friendly (can sequential writes within a single zone be reordered ?) is asking
>> for troubles (unaligned write errors showing up).
> 
> Ah, that is interesting.
> 
> Which device driver files are we talking about here, specifically?
> I'd like to take a look.
> 
> I guess what you say is not that you are looking for the deadline
> scheduling per se (as in deadline scheduling is nice), what you want is
> the zone locking semantics in that scheduler, is that right?
> 
> I.e. this business:
> blk_queue_is_zoned(q)
> blk_req_zone_write_lock(rq);
> blk_req_zone_write_unlock(rq);
> and mq-deadline solves this with a spinlock.
> 
> I will augment the patch to enforce mq-deadline
> if blk_queue_is_zoned(q) is true, as it is clear that
> any device with that characteristic must use mq-deadline.
> 
> Paoly might be interested in looking into whether BFQ could
> also handle zoned devices in the future, I have no idea of how
> hard that would be.
> 

Absolutely, as I already wrote in my reply to Damien.

In the meantime, Linus, augmenting your patch as you propose seems
a clean and effective solution to me.

Thanks,
Paolo

> The zoned business seems a bit fragile. Should it even be
> allowed to select any other scheduler than deadline on these
> devices? Presenting all compiled in schedulers in
> /sysblock/device/queue/scheduler sounds like just giving
> sysadmins too much rope.
> 
> Yours,
> Linus Walleij