Zram

zram, formerly called compcache, is a Linux kernel module for creating a compressed block device in RAM, i.e. a RAM disk with on-the-fly disk compression. The block device created with zram can then be used for swap or as a 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"). When empty, zram block device allocates about 0.1% of its size. 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 multiple compression algorithms. Compression algorithms include DEFLATE (DEFLATE), LZ4 (LZ4, and LZ4HC "high compression"), LZO (LZO-RLE "run-length encoding"), Zstandard (ZSTD), 842 (842). From kernel 5.1, the default is LZO-RLE, which has a balance of speed and compression ratio. Like most other system parameters, the compression algorithm can be selected via sysfs. When used as a compressed swap space, zram is similar to zswap, which is not a general-purpose RAM disk, but rather an in-kernel compressed cache for swap pages. However, zswap always requires a backing store, which is not the case for zram. Since the introduction of CONFIG_ZRAM_WRITEBACK in kernel version 4.14, zram is also able to use a storage device as a backing store, so it is able to move less-frequently used pages to disk, albeit the device can be only a partition at the moment (and the process cannot be managed automatically by kernel when the memory needs to be freed, but requires pre-emptive user-space programming). Since kernel version 7.0, zram can send the data to the...