From: Arnd Bergmann <arnd@arndb.de>
To: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@infradead.org>,
        Paolo Valente <paolo.valente@linaro.org>, Jens Axboe <axboe@kernel.dk>,
        Tejun Heo <tj@kernel.org>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, ulf.hansson@linaro.org,
        linus.walleij@linaro.org, broonie@kernel.org, hare@suse.de,
        grant.likely@secretlab.ca, James.Bottomley@hansenpartnership.com
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler
Date: Wed, 26 Oct 2016 17:13:07 +0200
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On Wednesday, October 26, 2016 8:05:11 AM CEST Bart Van Assche wrote:
> On 10/26/2016 04:34 AM, Jan Kara wrote:
> > On Wed 26-10-16 03:19:03, Christoph Hellwig wrote:
> >> Just as last time:
> >>
> >> big NAK for introducing giant new infrastructure like a new I/O scheduler
> >> for the legacy request structure.
> >>
> >> Please direct your engergy towards blk-mq instead.
> >
> > Christoph, we will probably talk about this next week but IMO rotating
> > disks and SATA based SSDs are going to stay with us for another 15 years,
> > likely more. For them blk-mq is no win, relatively complex IO scheduling
> > like CFQ or BFQ does is a big win for them in some cases. So I think IO
> > scheduling (and thus place for something like BFQ) is going to stay with us
> > for quite a long time still. So are we going to add hooks in blk-mq to
> > support full-blown IO scheduling at least for single queue devices? Or how
> > else do we want to support that HW?
> 
> Hello Jan,
> 
> Having two versions (one for non-blk-mq, one for blk-mq) of every I/O 
> scheduler would be a maintenance nightmare. Has anyone already analyzed 
> whether it would be possible to come up with an API for I/O schedulers 
> that makes it possible to use the same I/O scheduler for both blk-mq and 
> the traditional block layer?

The question to ask first is whether to actually have pluggable
schedulers on blk-mq at all, or just have one that is meant to
do the right thing in every case (and possibly can be bypassed
completely).

	Arnd