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Chapter 7. Block I/O

This chapter covers optimizing I/O settings in virtualized environments.

7.1. Block I/O Tuning

The virsh blkiotune command allows administrators to set or display a guest virtual machine's block I/O parameters manually in the <blkio> element in the guest XML configuration.
To display current <blkio> parameters for a virtual machine:
# virsh blkiotune virtual_machine
To set a virtual machine's <blkio> parameters, use the virsh blkiotune command and replace option values according to your environment:

# virsh blkiotune virtual_machine [--weight number] [--device-weights string] [--config] [--live] [--current]
Parameters include:
The I/O weight, within the range 100 to 1000.
Increasing the I/O weight of a device increases its priority for I/O bandwidth, and therefore provides it with more host resources. Similarly, reducing a device’s weight makes it consume less host resources.
A single string listing one or more device/weight pairs, in the format of /path/to/device,weight,/path/to/device,weight. Each weight must be within the range 100-1000, or the value 0 to remove that device from per-device listings. Only the devices listed in the string are modified; any existing per-device weights for other devices remain unchanged.
Add the --config option for changes to take effect at next boot.
Add the --live option to apply the changes to the running virtual machine.


The --live option requires the hypervisor to support this action. Not all hypervisors allow live changes of the maximum memory limit.
Add the --current option to apply the changes to the current virtual machine.
For example, the following changes the weight of the /dev/sda device in the liftbrul VM to 500.
# virsh blkiotune liftbrul --device-weights /dev/sda, 500


Use the virsh help blkiotune command for more information on using the virsh blkiotune command.