Understanding the Linux Kernel, Second Edition

Swap Area

The pages swapped out from memory are stored in a swap area , which may be implemented either as a disk partition of its own or as a file included in a larger partition. Several different swap areas may be defined, up to a maximum number specified by the MAX_SWAPFILES macro (usually set to 32).

Having multiple swap areas allows a system administrator to spread a lot of swap space among several disks so that the hardware can act on them concurrently; it also lets swap space be increased at runtime without rebooting the system.

Each swap area consists of a sequence of page slots : 4,096-byte blocks used to contain a swapped-out page. The first page slot of a swap area is used to persistently store some information about the swap area; its format is described by the swap_header union composed of two structures, info and magic. The magic structure provides a string that marks part of the disk unambiguously as a swap area; it consists of just one field, magic.magic, which contains a 10-character “magic” string. The magic structure essentially allows the kernel to unambiguously identify a file or a partition as a swap area; the text of the string depends on the swapping algorithm version: SWAP-SPACE for Version 1 or SWAPSPACE2 for Version 2. The field is always located at the end of the first page slot.

The info structure includes the following fields:

info.bootbits

Not used by the swapping algorithm; this field corresponds to the first 1,024 bytes of the swap area, which ...