Zram is a Linux kernel module that allows you to set up compressed filesystems in RAM.

zram-tools uses this module to set up compressed swap space. This is useful on systems with low memory or servers running a large amount of services with data that's easily swappable but that you may wish to swap back fast without sacrificing disk bandwidth.

By default it allocates 100MB of RAM, you can configure this in /etc/default/zramswap.