Zswap
| Developer(s) | Seth Jennings and others |
|---|---|
| Written in | C |
| Operating system | Linux |
| Type | Linux kernel features |
| License | GNU GPL |
| Website | kernel |
zswap is a Linux kernel feature that provides a compressed write-back cache for swapped pages, as a form of virtual memory compression. Instead of moving memory pages to a swap device when they are to be swapped out, zswap performs their compression and then stores them into a memory pool dynamically allocated in the system RAM. Later writeback to the actual swap device is deferred or even completely avoided, resulting in a significantly reduced I/O for Linux systems that require swapping; the tradeoff is the need for additional CPU cycles to perform the compression.
As a result of reduced I/O, zswap offers advantages to various devices that use flash-based storage, including embedded devices, netbooks and similar low-end hardware devices, as well as to other devices that use solid-state drives (SSDs) for storage. Flash memory has a limited lifespan due to its nature, so avoiding it to be used for providing swap space prevents it from wearing out quickly.
zswap is integrated into the rest of Linux kernel's virtual memory subsystem using the API provided by frontswap, which is a mechanism of the Linux kernel that abstracts various types of storage that can be used as swap space. As a result, zswap operates as a backend driver for frontswap by providing what is internally visible as a pseudo-RAM device. In other words, the frontswap API makes zswap capable of intercepting memory pages while they are being swapped out, and capable of intercepting page faults for the already swapped pages; the access to those two paths allows zswap to act as a compressed write-back cache for swapped pages.
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