1 zram: Compressed RAM based block devices
2 ----------------------------------------
6 The zram module creates RAM based block devices named /dev/zram<id>
7 (<id> = 0, 1, ...). Pages written to these disks are compressed and stored
8 in memory itself. These disks allow very fast I/O and compression provides
9 good amounts of memory savings. Some of the usecases include /tmp storage,
10 use as swap disks, various caches under /var and maybe many more :)
12 Statistics for individual zram devices are exported through sysfs nodes at
17 Following shows a typical sequence of steps for using zram.
20 modprobe zram num_devices=4
21 This creates 4 devices: /dev/zram{0,1,2,3}
22 (num_devices parameter is optional. Default: 1)
24 2) Set max number of compression streams
25 Compression backend may use up to max_comp_streams compression streams,
26 thus allowing up to max_comp_streams concurrent compression operations.
27 By default, compression backend uses single compression stream.
30 #show max compression streams number
31 cat /sys/block/zram0/max_comp_streams
33 #set max compression streams number to 3
34 echo 3 > /sys/block/zram0/max_comp_streams
37 In order to enable compression backend's multi stream support max_comp_streams
38 must be initially set to desired concurrency level before ZRAM device
39 initialisation. Once the device initialised as a single stream compression
40 backend (max_comp_streams equals to 0) changing the value of max_comp_streams
41 will not take any effect, because single stream compression backend implemented
42 as a special case and does not support dynamic max_comp_streams. Only multi
43 stream backend supports dynamic max_comp_streams adjustment.
45 3) Select compression algorithm
46 Using comp_algorithm device attribute one can see available and
47 currently selected (shown in square brackets) compression algortithms,
48 change selected compression algorithm (once the device is initialised
49 there is no way to change compression algorithm).
52 #show supported compression algorithms
53 cat /sys/block/zram0/comp_algorithm
56 #select lzo compression algorithm
57 echo lzo > /sys/block/zram0/comp_algorithm
60 Set disk size by writing the value to sysfs node 'disksize'.
61 The value can be either in bytes or you can use mem suffixes.
63 # Initialize /dev/zram0 with 50MB disksize
64 echo $((50*1024*1024)) > /sys/block/zram0/disksize
67 echo 256K > /sys/block/zram0/disksize
68 echo 512M > /sys/block/zram0/disksize
69 echo 1G > /sys/block/zram0/disksize
72 There is little point creating a zram of greater than twice the size of memory
73 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
74 size of the disk when not in use so a huge zram is wasteful.
84 Per-device statistics are exported as various nodes under
102 Write any positive value to 'reset' sysfs node
103 echo 1 > /sys/block/zram0/reset
104 echo 1 > /sys/block/zram1/reset
106 This frees all the memory allocated for the given device and
107 resets the disksize to zero. You must set the disksize again
108 before reusing the device.