Documentation/blockdev/zram.txt

   1 zram: Compressed RAM based block devices
   2 ----------------------------------------
   3
   4 * Introduction
   5
   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 :)
  11
  12 Statistics for individual zram devices are exported through sysfs nodes at
  13 /sys/block/zram<id>/
  14
  15 * Usage
  16
  17 Following shows a typical sequence of steps for using zram.
  18
  19 1) Load Module:
  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)
  23
  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.
  28
  29         Examples:
  30         #show max compression streams number
  31         cat /sys/block/zram0/max_comp_streams
  32
  33         #set max compression streams number to 3
  34         echo 3 > /sys/block/zram0/max_comp_streams
  35
  36 Note:
  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 1), you will see error if you try to change
  41 the value of max_comp_streams because single stream compression backend
  42 implemented as a special case by lock overhead issue and does not support
  43 dynamic max_comp_streams. Only multi stream backend supports dynamic
  44 max_comp_streams adjustment.
  45
  46 3) Select compression algorithm
  47         Using comp_algorithm device attribute one can see available and
  48         currently selected (shown in square brackets) compression algortithms,
  49         change selected compression algorithm (once the device is initialised
  50         there is no way to change compression algorithm).
  51
  52         Examples:
  53         #show supported compression algorithms
  54         cat /sys/block/zram0/comp_algorithm
  55         lzo [lz4]
  56
  57         #select lzo compression algorithm
  58         echo lzo > /sys/block/zram0/comp_algorithm
  59
  60 4) Set Disksize
  61         Set disk size by writing the value to sysfs node 'disksize'.
  62         The value can be either in bytes or you can use mem suffixes.
  63         Examples:
  64             # Initialize /dev/zram0 with 50MB disksize
  65             echo $((50*1024*1024)) > /sys/block/zram0/disksize
  66
  67             # Using mem suffixes
  68             echo 256K > /sys/block/zram0/disksize
  69             echo 512M > /sys/block/zram0/disksize
  70             echo 1G > /sys/block/zram0/disksize
  71
  72 Note:
  73 There is little point creating a zram of greater than twice the size of memory
  74 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
  75 size of the disk when not in use so a huge zram is wasteful.
  76
  77 5) Set memory limit: Optional
  78         Set memory limit by writing the value to sysfs node 'mem_limit'.
  79         The value can be either in bytes or you can use mem suffixes.
  80         In addition, you could change the value in runtime.
  81         Examples:
  82             # limit /dev/zram0 with 50MB memory
  83             echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
  84
  85             # Using mem suffixes
  86             echo 256K > /sys/block/zram0/mem_limit
  87             echo 512M > /sys/block/zram0/mem_limit
  88             echo 1G > /sys/block/zram0/mem_limit
  89
  90             # To disable memory limit
  91             echo 0 > /sys/block/zram0/mem_limit
  92
  93 6) Activate:
  94         mkswap /dev/zram0
  95         swapon /dev/zram0
  96
  97         mkfs.ext4 /dev/zram1
  98         mount /dev/zram1 /tmp
  99
 100 7) Stats:
 101         Per-device statistics are exported as various nodes under
 102         /sys/block/zram<id>/
 103                 disksize
 104                 num_reads
 105                 num_writes
 106                 failed_reads
 107                 failed_writes
 108                 invalid_io
 109                 notify_free
 110                 zero_pages
 111                 orig_data_size
 112                 compr_data_size
 113                 mem_used_total
 114                 mem_used_max
 115
 116 8) Deactivate:
 117         swapoff /dev/zram0
 118         umount /dev/zram1
 119
 120 9) Reset:
 121         Write any positive value to 'reset' sysfs node
 122         echo 1 > /sys/block/zram0/reset
 123         echo 1 > /sys/block/zram1/reset
 124
 125         This frees all the memory allocated for the given device and
 126         resets the disksize to zero. You must set the disksize again
 127         before reusing the device.
 128
 129 Nitin Gupta
 130 ngupta@vflare.org