Hi,

Following VMware recommendations on how to change linux I/O scheduler for guests, I'm trying to do it on my OMV VM machine running Debian Wheezy.

First, lets check which I/O scheduler is configured:


As we can see above for device /dev/sda, the CFQ (Completely Fair Queueing) is the default scheduler that's being used, the one selected in brackets. Since the recommendation for VMware Virtual Machines is to change the scheduler to "noop" (as the hypervisor has its I/O scheduler), we can change it on the fly:


This change is only temporary and will reset back to the default scheduler, CFQ in this case, when the machine reboots. To make the setting permanently, I've tried to change the grub configuration file by editing /etc/default/grub and add "elevator=noop" setting to GRUB_CMDLINE_LINUX_DEFAULT parameter, like below :


I could check that the new setting was added to /boot/grub/grub.cfg:


The problem is that after system restart, the CFQ I/O scheduler is still active for /dev/sda:


I've done the same configuration on Ubuntu Server 14.04 and CentOS 7 and everything worked fine after boot.
Am I missing something?

Thanks.

Hi,

Yes, I've run update-grub2 (which is a soft link to update-grub) as stated in my post.
I know I can get a workaround to change the I/O scheduler, but I want to understand why changing grub did not worked out.

Thx.

Hi,

I think I've found why editing grub default did not worked. It seems that OMV has placed a file in /etc/udev/rules.d called 99-openmediavault-scheduler.rules



As we can see above, this udev rule is forcing all detected HDD with rotational=1 (either hdX or sdX) to change the scheduler to "cfq". Since VMDK disks on VMware are detected as rotational disks, my edit on grub was being overwritten by this udev rule.

In OMV, to change the I/O scheduler permanently for virtual disks , we only need to edit this rule and change the scheduler to "noop", like this:


After boot, scheduler was changed like I wanted to.

Hope this will help other users.

Thx