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In kernel 3.18RC1 this kernel config parameter is: # CONFIG_SCSI_MQ_DEFAULT is not set
In kernel 3.18RC2 and beyond, the kernel config parameter is: CONFIG_SCSI_MQ_DEFAULT=y

This results in loss of the ability to set the IO scheduler via
/sys/block/sda/queue/scheduler.

Now we get:

doug@s15:~$ cat /sys/block/sda/queue/scheduler
none

Where we are used to getting:

doug@s15:~/temp2$ cat /sys/block/sda/queue/scheduler
noop [deadline] cfq

>From the add a CONFIG_SCSI_MQ_DEFAULT option commit message:

> Add a Kconfig option to enable the blk-mq path for SCSI by default
> to ease testing and deployment in setups that know they benefit
> from blk-mq.

How do we know that all systems benifit from blk-mq?

It seems complicated to have to re-compile the kernel to get the other IO scheduler options back.
Why isn't this option done similar to the others? I.E.

doug@s15:~/temp2$ cat /sys/block/sda/queue/scheduler
noop [deadline] cfq blk-mq

(and I realize that is actually an upstream question.)

By the way, my system does seem to benefit from blk-mq, I just didn't
understand why I couldn't observe and change the IO scheduler anymore,
and so isolated the change.

Experimental data:

Random seeks in a large file:
blk-mq: 104 seeks per second average
deadline: 74 seeks per second average
cfq: 74 seeks per second average
noop: 74 seeks per second average

Kernel compile:
deadline: 23 minutes 37.4 seconds
blk-mq: 23 minutes 35.4 seconds

Note 1: Please do not ask for all of my apport stuff, it is not needed for this bug report.
Note 2: on IRC "apw" asked me to enter this bug report

** Affects: linux (Ubuntu)
     Importance: Undecided
     Assignee: Andy Whitcroft (apw)
         Status: Triaged

-- 
Kernel Config setting: CONFIG_SCSI_MQ_DEFAULT
https://bugs.launchpad.net/bugs/1397061
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