ZRAM is the latest version of a RAM disk: a virtual filesystem created using RAM. The idea is as old as personal computers, and in the early days, when there was a limit of 640k RAM, extra memory was often made into a RAM disk to improve performance. Originally called compcache, zRAM is a kernel module that creates a compressed device in RAM, usually for use as a swap device. This use is so efficient that, in the past few years, with little attention, zRAM has been enabled during installation in many distributions.

Swap, if you need to be reminded, is device memory that is used when all available RAM is used. On most machines, swap is allocated to a specially formatted partition, listed in /etc/fstab along with regular partitions, although it can also be a single file made by the root user with the command mkswap /swapfile. Multiple swap sources are possible, so if a swap partition proves too small, a swap file can be added in much less time than resizing the swap partition would take. You can choose which swap source to use first by adjusting priorities (see below).

Any sort of swap memory can keep your computer running when all the RAM is allocated, although these days, when home computers often have 8GB or more of RAM, you may only need it if you are doing intensive work such as graphic design, or if you regularly hibernate your machine. The only trouble is, device memory is glacially slow compared to RAM: .10 percent of RAM speed if it is on an SSD drive and .01 percent if it is on a mechanical hard drive. For this reason, RAM disks have always been popular on computers, although their use was impractical until about two decades ago, when the higher amounts of RAM became the norm. Today, zRAM's added compression means that more swap is available with the same amount of RAM.

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