admin-guide/blockdev/zram.rst

2 zram: Compressed RAM-based block devices
8 The zram module creates RAM-based block devices named /dev/zram<id>
14 Statistics for individual zram devices are exported through sysfs nodes at
15 /sys/block/zram<id>/
20 There are several ways to configure and manage zram device(-s):
22 a) using zram and zram_control sysfs attributes
25 In this document we will describe only 'manual' zram configuration steps,
26 IOW, zram and zram_control sysfs attributes.
30 that zram maintainers do not develop/maintain util-linux or zramctl, should
33 Following shows a typical sequence of steps for using zram.
41 zram sysfs attributes always return negative values in case of errors.
47 -ENOMEM	  zram was not able to allocate enough memory to fulfil your
67 	modprobe zram num_devices=4
69 This creates 4 devices: /dev/zram{0,1,2,3}
71 num_devices parameter is optional and tells zram how many devices should be
131 There is little point creating a zram of greater than twice the size of memory
132 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
133 size of the disk when not in use so a huge zram is wasteful.
165 7) Add/remove zram devices
168 zram provides a control interface, which enables dynamic (on-demand) device
173 can use /dev/zram<id>) or an error code.
177 	cat /sys/class/zram-control/hot_add
183 	echo X > /sys/class/zram-control/hot_remove
188 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
191 please read Documentation/ABI/testing/sysfs-block-zram.
202 writeback_limit   	WO	specifies the maximum amount of write IO zram
209 debug_stat        	RO	this file is used for zram debugging purposes
210 backing_dev	  	RW	set up backend storage for zram to write out
217 File /sys/block/zram<id>/stat
222 File /sys/block/zram<id>/io_stat
225 layer and, thus, not available in zram<id>/stat file. It consists of a
247 File /sys/block/zram<id>/mm_stat
263  mem_used_max     the maximum amount of memory zram has consumed to
271 File /sys/block/zram<id>/bd_stat
311 With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
322 To use idle page writeback, first, user need to declare zram pages
327 From now on, any pages on zram are idle pages. The idle mark
335 With the command, zram writeback idle pages from memory to the storage.
337 If admin want to write a specific page in zram device to backing device,
346 To overcome the concern, zram supports "writeback_limit" feature.
353 Once writeback_limit_enable is set, zram doesn't allow any writeback
382 The writeback_limit count will reset whenever you reset zram (e.g.,
384 writeback happened until you reset the zram to allocate extra writeback
394 zram block. It could be useful to catch cold or incompressible
398 /sys/kernel/debug/zram/zram0/block_state". The output is as follows::
405 	zram's block index.