KVM and I/O problems

A friend of my helped me out. The problem is the writing scheduler. After i know this I found in a post here, followed by the first link:

This approach is great, but the fatal flaw is that it assumes a single, physical disk, attached to a single physical SCSI controller in a single physical host. How does the elevator algorithm know what to do when the disk is actually a RAID array? Does it? Or, what if that one Linux kernel isn’t the only kernel running on a physical host? Does the elevator mechanism still help in virtual environments? No, no it doesn’t. Hypervisors have elevators, too. So do disk arrays. Remember that in virtual environments the hypervisor can’t tell what is happening inside the VM[0]. It’s a black box, and all it sees is the stream of I/O requests that eventually get passed to the hypervisor. It doesn’t know if they got reordered, how they got reordered, or why. It doesn’t know how long the request has been outstanding. As a result it probably won’t make the best decision about handling those requests, adding latency and extra work for the array. Worst case, all that I/O ends up looking very random to the disk array.

Fixing this using the io scheduler of the SCSI HDD/Raid 1. Disabling it via on a live system.

echo noop > /sys/block/queue/scheduler

Enabling it into the system by editing /etc/grub.conf with option

elevator=noop

 

Sources:

http://www.gnutoolbox.com/linux-io-elevator/

http://blog.bodhizazen.net/linux/improve-kvm-performance/

http://lonesysadmin.net/2008/02/21/elevatornoop/

https://www.redhat.com/magazine/008jun05/features/schedulers/