admin-guide/blockdev/zram.rst

10 in memory itself. These disks allow very fast I/O and compression provides
74 2) Set max number of compression streams
78 allocate multiple compression streams - one per online CPU - thus
79 allowing several concurrent compression operations. The number of
80 allocated compression streams goes down when some of the CPUs
81 become offline. There is no single-compression-stream mode anymore,
88 3) Select compression algorithm
92 currently selected (shown in square brackets) compression algorithms,
93 or change the selected compression algorithm (once the device is initialised
94 there is no way to change compression algorithm).
98 	#show supported compression algorithms
102 	#select lzo compression algorithm
105 For the time being, the `comp_algorithm` content shows only compression
108 4) Set compression algorithm parameters: Optional
111 Compression algorithms may support specific parameters which can be
115 For example, several compression algorithms support `level` parameter.
116 In addition, certain compression algorithms support pre-trained dictionaries,
118 compression algorithm to use external pre-trained dictionary, pass full
128 	#pass path to pre-trained zstd dictionary and compression level
134 algorithms `level` controls the compression level (the higher the value the
135 better the compression ratio, it even can take negatives values for some
137 the value the lower the compression ratio).
156 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
231 comp_algorithm    	RW	show and change the compression algorithm
232 algorithm_params	WO	setup compression algorithm parameters
438 (secondary) compression algorithms. The basic idea is that alternative
439 compression algorithm can provide better compression ratio at a price of
440 (potentially) slower compression/decompression speeds. Alternative compression
446 With CONFIG_ZRAM_MULTI_COMP, zram supports up to 4 compression algorithms:
448 in "3) Select compression algorithm", secondary algorithms are configured
458 Alternative compression algorithms are sorted by priority. In the example
460 of 1, while lz4hc is configured as a compression algorithm with priority 2.
461 Alternative compression algorithm's priority is provided during algorithms
494 It is also possible to limit the number of pages zram re-compression will
501 During re-compression for every page, that matches re-compression criteria,
502 ZRAM iterates the list of registered alternative compression algorithms in
503 order of their priorities. ZRAM stops either when re-compression was
505 and matches re-compression criteria (e.g. size threshold) or when there are
510 This re-compression behaviour, when it iterates through the list of
511 registered compression algorithms, increases our chances of finding the
555 		recompressed page (secondary compression algorithm)