7.4. Block I/O Tuning Techniques

When several virtual machines are running simultaneously, they can interfere with system performance by using excessive disk I/O. Disk I/O throttling in KVM provides the ability to set a limit on disk I/O requests sent from virtual machines to the host machine. This can prevent a virtual machine from over-utilizing shared resources and impacting the performance of other virtual machines.

Disk I/O throttling can be useful in various situations, for example when guest virtual machines belonging to different customers are running on the same host, or when quality of service guarantees are given for different guests. Disk I/O throttling can also be used to simulate slower disks.

I/O throttling can be applied independently to each block device attached to a guest and supports limits on throughput and I/O operations. Use the virsh blkdeviotune command to set I/O limits for a virtual machine:

virsh blkdeviotune virtual_machine device --parameter limit

# virsh blkdeviotune virtual_machine device --parameter limit

Device specifies a unique target name (<target dev='name'/>) or source file (<source file='name'/>) for one of the disk devices attached to the virtual machine. Use the virsh domblklist command for a list of disk device names.

Optional parameters include:

For example, to throttle vda on virtual_machine to 1000 I/O operations per second and 50 MB per second throughput, run this command:

virsh blkdeviotune virtual_machine vda --total-iops-sec 1000 --total-bytes-sec 52428800

# virsh blkdeviotune virtual_machine vda --total-iops-sec 1000 --total-bytes-sec 52428800

Multi-queue virtio-scsi provides improved storage performance and scalability in the virtio-scsi driver. It enables each virtual CPU to have a separate queue and interrupt to use without affecting other vCPUs.

   <controller type='scsi' index='0' model='virtio-scsi'>
   <driver queues='N' />
    </controller>

   <controller type='scsi' index='0' model='virtio-scsi'>
   <driver queues='N' />
    </controller>