While creating a new kvm guest drive, a host system becomes extremely slow and unresponsive

What:

Linux host with kvm hypervisor

Problem:

While allocating space for a new kvm guest drive a host system becomes extremely slow and unresponsive, it brings entire infrastructure down.

Solution:

Use ionice.
If you are using SLES or Red hat it's probably already installed if your system is Debian or Ubuntu you may need to install it with.

apt-get install ionice

Find your process id via top or ps aux
Use following command to change priority

sudo ionice -c3 -p<pid>

where:
-c3 places the process in the idle scheduling class
-p your process id (pid)

It might be a better idea to change priority for a whole shell using following command.

sudo ionice -c3 -p$$

 Echo $$ to check your shell id

echo $$