memory ballooning

Memory ballooning allows you to have your guest dynamically change it’s memory usage by evicting unused memory during runtime. This is a useful feature because it reduces the impact your guest can have on memory usage of your host by giving up unused memory back to the host.

Certain guests (only Linux at the moment) have a balloon driver, so the host can have the guest allocate a certain amount of memory which the guest won't be able to use anymore and it can then be freed on the host.

Below command gives the information regarding free memory. Using balloon command you can change guest machine to change it’s memory allocation to the specified amount in MB which is 800MB in the below example.

(qemu) info balloon

The syntax for the command to actually perform memory ballooning is as follows.

(qemu) balloon 800