** Changed in: linux (Ubuntu)
Importance: Undecided => High
** Changed in: linux (Ubuntu)
Status: Triaged => Fix Committed
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https://bugs.launchpad.net/bugs/1397061
Title:
CONFIG_SCSI_MQ_DEFAULT default changed preventing use of IO schedulers
Status in “linux” package in Ubuntu:
Fix Committed
Bug description:
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
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