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Bug#914758: linux-image-4.18.0-2-amd64: setting elevator=none does not change io scheduler

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Control: tag -1 upstream
Control: retitle -1 Missing equivalent of elevator kernel parameter for blk-mq

On Mon, 2018-11-26 at 18:10 -0600, Matt Zagrabelny wrote:
> Package: src:linux
> Version: 4.18.10-2
> Severity: normal
> 
> Greetings!
> 
> Thank you for your contribution to Debian. It is appreciated!
> 
> I am running:
> 
> linux-image-4.18.0-2-amd64 4.18.10-2+b1
> 
> and attempting to set the IO scheduler.
> 
> Looking at the current scheduler:
> 
> # cat /sys/block/sda/queue/scheduler
> [mq-deadline] none

This is a device implemented using the "blk-mq" API, which was
introduced a few years ago but has only recently come to be used by
most block devices.

[...]
> I just don't know why the elevator parameter isn't working. According to the
> kernel documentation for 4.18:
> 
> grep -A3 elevator /usr/src/linux-source-4.18/Documentation/admin-guide/kernel-parameters.txt
> elevator=       [IOSCHED]
>                 Format: {"cfq" | "deadline" | "noop"}
>                 See Documentation/block/cfq-iosched.txt and
>                 Documentation/block/deadline-iosched.txt for details.
[...]

This is correct, but incomplete.  The "elevator" kernel parameter only
controls the default schedulers for the old "blk-rq" device API.

Unfortunately, because of the differences between the old and new APIs
it was also necessary to introduce a new set of schedulers.  There
isn't (yet) a kernel parameter to choose the default for blk-mq
devices.

Ben.

-- 
Ben Hutchings
Beware of bugs in the above code;
I have only proved it correct, not tried it. - Donald Knuth

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Bug#914758: linux-image-4.18.0-2-amd64: setting elevator=none does not change io scheduler Matt Zagrabelny <mzagrabe@d.umn.edu> - 2018-11-27 01:20 +0100
  Bug#914758: linux-image-4.18.0-2-amd64: setting elevator=none does not change io scheduler Ben Hutchings <ben@decadent.org.uk> - 2018-11-27 04:00 +0100
  Processed: Re: Bug#914758: linux-image-4.18.0-2-amd64: setting  elevator=none does not change io scheduler "Debian Bug Tracking System" <owner@bugs.debian.org> - 2018-11-27 04:00 +0100

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