11.1. What is Swap Space?

Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory.

Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files.

The size of your swap should be equal to twice your computer's physical RAM for up to 2 GB of physical RAM. For physical RAM above 2 GB, the size of your swap should be equal to the amount of physical RAM above 2 GB. The size of your swap should never less than 32 MB.

Using this basic formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one with 3 GB of physical RAM would have 5 GB of swap.

	Note
	Unfortunately, deciding on the amount of swap to allocate to Red Hat Enterprise Linux is more of an art than a science, so hard rules are not possible. Each system's most used applications should be accounted for when determining swap size.

Important

File systems and LVM2 volumes assigned as swap space cannot be in use when being modified. For example, no system processes can be assigned the swap space, as well as no amount of swap should be allocated and used by the kernel. Use the free and cat /proc/swaps commands to verify how much and where swap is in use. The best way to achieve swap space modifications is to boot your system in rescue mode, and then follow the instructions (for each scenario) in the remainder of this chapter. Refer to Chapter 5 Basic System Recovery for instructions on booting into rescue mode. When prompted to mount the file system, select Skip.