Comment 7 for bug 1008400

@Andy - The logs that could be interesting are the logs of the virtual server (kvm). However I don't have this log anymore, as I had to replace the kvm image with a backup because of this problem. But let me try to explain the situation, you'll see that unfairness is not the issue here.

- I installed a new Ubuntu 12.04 server (host), with CFQ as scheduler (new default)
- A kvm image was placed on the host (local disks, software raid), and it was started. I call this virtual server 'kvm'.
- The kvm was running fine all day, no problems at all
- At night some big files were rsync'd to the host, with normal priorities.
- A few minutes after the rsync started the kvm started logging ata disk timeouts. After some time the disks were remounted read-only due to these errors.
- The next day I tried to restart the kvm, but the image was completely corrupted.
- After placing back a backup the kvm was started again
- The next night exactly the same happened during the rsync.
- The next day I placed back the backup again, and changed the scheduler to 'deadline' (old default). This solved the problem.

But this is just one of many examples of what can go wrong with CFQ. Internet is full of stories about CFQ causing very slow i/o or worse for several common server tasks. I really think the default scheduler should be changed to deadline again for servers. It is not without reason that the old -server kernels did this.