linux-kernel - Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

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Date:   Wed, 26 Oct 2016 09:32:36 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@...radead.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Bart Van Assche <bart.vanassche@...disk.com>,
        Jan Kara <jack@...e.cz>,
        Paolo Valente <paolo.valente@...aro.org>,
        Tejun Heo <tj@...nel.org>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, ulf.hansson@...aro.org,
        linus.walleij@...aro.org, broonie@...nel.org, hare@...e.de,
        grant.likely@...retlab.ca, James.Bottomley@...senpartnership.com
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra
 scheduler

On 10/26/2016 09:29 AM, Christoph Hellwig wrote:
> On Wed, Oct 26, 2016 at 05:13:07PM +0200, Arnd Bergmann wrote:
>> 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).
>
> That would be my preference.  Have a BFQ-variant for blk-mq as an
> option (default to off unless opted in by the driver or user), and
> not other scheduler for blk-mq.  Don't bother with bfq for non
> blk-mq.  It's not like there is any advantage in the legacy-request
> device even for slow devices, except for the option of having I/O
> scheduling.

It's the only right way forward. blk-mq might not offer any substantial
advantages to rotating storage, but with scheduling, it won't offer a
downside either. And it'll take us towards the real goal, which is to
have just one IO path. Adding a new scheduler for the legacy IO path
makes no sense. Adding one for blk-mq and phasing out the old path is
what we need to do.

-- 
Jens Axboe