zram

This article is about the Linux kernel feature. For the memory hardware technology, seeZ-RAM.For the Linux kernel compressed swap cache, seezswap.

zram

Initial release	March 30, 2014(2014-03-30)(Linux 3.14)
Written in	C
Operating system	Linux
Type	Linux kernel features
License	GNU General Public License
Website	www.kernel.org

zram,formerly calledcompcache,is aLinux kernel modulefor creating a compressedblock devicein RAM, i.e. aRAM diskwith on-the-fly disk compression. The block device created with zram can then be used forswapor as general-purpose RAM disk. The two most common uses for zram are for the storage of temporary files (/tmp) and as a swap device. Initially, zram had only the latter function, hence the original name "compcache" ( "compressed cache" ). Unlike swap, zram only uses 0.1% of the maximum size of the disk when not in use.

After four years in the Linux kernel's driver staging area, zram was introduced into the mainline Linux kernel in version 3.14, released on March 30, 2014.From Linux kernel version 3.15 onwards (released on June 8, 2014), zram supports multiple compression streams and multiplecompression algorithms.Compression algorithms includeDEFLATE(DEFLATE),LZ4(LZ4,andLZ4HC"high compression" ),LZO(LZO-RLE"run-length encoding" ),Zstandard(ZSTD),842(842). From kernel 5.1, the default isLZO-RLE,which has a balance of speed and compression ratio. Like most other system parameters, the compression algorithm can be selected viasysfs.

When used as a compressed swap space, zram is similar tozswap,which is not a general-purpose RAM disk, but rather an in-kernel compressed cache for swap pages. Until the introduction ofCONFIG_ZRAM_WRITEBACKin kernel version 4.14, unlike zswap, zram was unable to use a storage device as a backing store, so it was unable to move less-frequently used pages to disk. However, zswap always requires a backing store, which is not the case for zram.

When used for swap, zram (like zswap) allows Linux to make more efficient use of RAM, since the operating system can then hold more pages of memory in the compressed swap than if the same amount of RAM had been used as application memory or disk cache. This is particularly effective on machines that do not have much memory.In 2012,Ubuntubriefly considered enabling zram by default on computers with small amounts of installed RAM.For this same reason,Fedoraenabled zram by default starting with release 33.

Using compressed swap space with zram or zswap also offers advantages for low-end hardware devices such asembedded devicesandnetbooks.Such devices usually useflash-based storage,which has limited lifespan due towrite amplification,and may also use it to provideswap space.Using zram or zswap reduces the swap usage, which effectively reduces the amount of wear placed on flash-based storage and makes it last longer. Using zram also results in significantly reducedI/Ofor Linux systems that require swapping.

See also

• Swap partitions on SSDs
• Virtual memory compression

External links

• zramLinux kernel documentation andzramctl(8)manual page
• Compcache, Compressed Caching for Linux
• Compcache: in-memory compressed swapping,May 26, 2009, LWN.net, by Nitin Gupta
• In-kernel memory compression,April 3, 2013, LWN.net, by Dan Magenheimer
• The Compression Cache: Virtual Memory Compression for Handheld Computers,March 16, 2000, byMichael J. Freedman