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I've created a RAID 5 device block by combining 4 HDDs.

The name of my device is md1.

But the /sys/block/md1/queue/scheduler file outputs "none" on doing cat.

And also after editing it using vim its throwing

/sys/devices/virtual/block/md1/queue/scheduler" E667: Fsync failed

I also tried using sysctl.conf and by adding

block.md1.queue.scheduler = deadline

But again it throws

error: "block.md1.queue.scheduler" is an unknown key

I'm really out of options. Please help me out.

share|improve this question
Here be dragons! – Tom O'Connor Feb 7 '13 at 9:51
Which OS distribution/version/kernel are you using? That's a very pertinent detail! – ewwhite Feb 7 '13 at 9:54
I'm using CentOS 6.3 – Nilesh Feb 7 '13 at 9:56
IIRC scheduling is set on the physical devices (sda/sdb/sdc...) not the md dev – symcbean Feb 7 '13 at 10:07
@symcbean That thought came to my mind as well. But I was not sure about it that it can't be done. – Nilesh Feb 7 '13 at 10:19
up vote 5 down vote accepted

You can apply the I/O scheduler to the real block devices; e.g. your /dev/sdX devices or default a global append to the kernel boot command line in /etc/grub... (add elevator=deadline)

But since you're using EL6 (Red Hat, CentOS, etc.), the most effective approach would be to use the tuned framework.

In your situation,

yum install tuned tuned-utils

Then choose a profile which will provide the right setting for your application. For instance:

tuned-adm profile throughput-performance


tuned-adm profile enterprise-storage

Tuned chart

share|improve this answer
Thanks. I did it but still the scheduler file for RAID block outputs "none". – Nilesh Feb 7 '13 at 10:14
@Nilesh What did you do? – ewwhite Feb 7 '13 at 10:14
I installed and set the profile to throughput-performance and then checked the file /sys/block/md1/queue/scheduler – Nilesh Feb 7 '13 at 10:16
Look at the output of when you run tuned-adm profile throughput-performance. It will say something like: Applying deadline elevator: sda sdb sdc sdd [ OK ]. As I stated in the answer above, the elevator settings are applied to your block devices... your individual disks, NOT the mdX device! – ewwhite Feb 7 '13 at 10:18
Yeah that happened. And I cross checked the sdX scheduler file and they had that change. – Nilesh Feb 7 '13 at 10:20

print current scheduler for all physical disks

for d in $(find /sys -name scheduler | egrep '(/sd|/hd)'); do echo $d $(cat $d); done

cat /sys/devices/virtual/block/md1/queue/scheduler

[noop] deadline cfq

echo "deadline" > /sys/devices/virtual/block/md1/queue/scheduler

share|improve this answer
This might be somewhat useful, but it doesn't answer the question, and so it should be a comment. Also the explanation is insufficient (the title says "print", but the printing is done in the first line, the last one will modify scheduling). I won't downvote because nobody else mentioned the correct way to modify the scheduler (vim is not recommended). – Law29 Dec 22 '15 at 23:21

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