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
  23 num_devices parameter is optional and tells zram how many devices should be
  24 pre-created. Default: 1.
  25
  26 2) Set max number of compression streams
  27         Compression backend may use up to max_comp_streams compression streams,
  28         thus allowing up to max_comp_streams concurrent compression operations.
  29         By default, compression backend uses single compression stream.
  30
  31         Examples:
  32         #show max compression streams number
  33         cat /sys/block/zram0/max_comp_streams
  34
  35         #set max compression streams number to 3
  36         echo 3 > /sys/block/zram0/max_comp_streams
  37
  38 Note:
  39 In order to enable compression backend's multi stream support max_comp_streams
  40 must be initially set to desired concurrency level before ZRAM device
  41 initialisation. Once the device initialised as a single stream compression
  42 backend (max_comp_streams equals to 1), you will see error if you try to change
  43 the value of max_comp_streams because single stream compression backend
  44 implemented as a special case by lock overhead issue and does not support
  45 dynamic max_comp_streams. Only multi stream backend supports dynamic
  46 max_comp_streams adjustment.
  47
  48 3) Select compression algorithm
  49         Using comp_algorithm device attribute one can see available and
  50         currently selected (shown in square brackets) compression algortithms,
  51         change selected compression algorithm (once the device is initialised
  52         there is no way to change compression algorithm).
  53
  54         Examples:
  55         #show supported compression algorithms
  56         cat /sys/block/zram0/comp_algorithm
  57         lzo [lz4]
  58
  59         #select lzo compression algorithm
  60         echo lzo > /sys/block/zram0/comp_algorithm
  61
  62 4) Set Disksize
  63         Set disk size by writing the value to sysfs node 'disksize'.
  64         The value can be either in bytes or you can use mem suffixes.
  65         Examples:
  66             # Initialize /dev/zram0 with 50MB disksize
  67             echo $((50*1024*1024)) > /sys/block/zram0/disksize
  68
  69             # Using mem suffixes
  70             echo 256K > /sys/block/zram0/disksize
  71             echo 512M > /sys/block/zram0/disksize
  72             echo 1G > /sys/block/zram0/disksize
  73
  74 Note:
  75 There is little point creating a zram of greater than twice the size of memory
  76 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
  77 size of the disk when not in use so a huge zram is wasteful.
  78
  79 5) Set memory limit: Optional
  80         Set memory limit by writing the value to sysfs node 'mem_limit'.
  81         The value can be either in bytes or you can use mem suffixes.
  82         In addition, you could change the value in runtime.
  83         Examples:
  84             # limit /dev/zram0 with 50MB memory
  85             echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
  86
  87             # Using mem suffixes
  88             echo 256K > /sys/block/zram0/mem_limit
  89             echo 512M > /sys/block/zram0/mem_limit
  90             echo 1G > /sys/block/zram0/mem_limit
  91
  92             # To disable memory limit
  93             echo 0 > /sys/block/zram0/mem_limit
  94
  95 6) Activate:
  96         mkswap /dev/zram0
  97         swapon /dev/zram0
  98
  99         mkfs.ext4 /dev/zram1
 100         mount /dev/zram1 /tmp
 101
 102 7) Add/remove zram devices
 103
 104 zram provides a control interface, which enables dynamic (on-demand) device
 105 addition and removal.
 106
 107 In order to add a new /dev/zramX device, perform read operation on hot_add
 108 attribute. This will return either new device's device id (meaning that you
 109 can use /dev/zram<id>) or error code.
 110
 111 Example:
 112         cat /sys/class/zram-control/hot_add
 113         1
 114
 115 To remove the existing /dev/zramX device (where X is a device id)
 116 execute
 117         echo X > /sys/class/zram-control/hot_remove
 118
 119 8) Stats:
 120 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
 121
 122 A brief description of exported device attritbutes. For more details please
 123 read Documentation/ABI/testing/sysfs-block-zram.
 124
 125 Name            access            description
 126 ----            ------            -----------
 127 disksize          RW    show and set the device's disk size
 128 initstate         RO    shows the initialization state of the device
 129 reset             WO    trigger device reset
 130 num_reads         RO    the number of reads
 131 failed_reads      RO    the number of failed reads
 132 num_write         RO    the number of writes
 133 failed_writes     RO    the number of failed writes
 134 invalid_io        RO    the number of non-page-size-aligned I/O requests
 135 max_comp_streams  RW    the number of possible concurrent compress operations
 136 comp_algorithm    RW    show and change the compression algorithm
 137 notify_free       RO    the number of notifications to free pages (either
 138                         slot free notifications or REQ_DISCARD requests)
 139 zero_pages        RO    the number of zero filled pages written to this disk
 140 orig_data_size    RO    uncompressed size of data stored in this disk
 141 compr_data_size   RO    compressed size of data stored in this disk
 142 mem_used_total    RO    the amount of memory allocated for this disk
 143 mem_used_max      RW    the maximum amount memory zram have consumed to
 144                         store compressed data
 145 mem_limit         RW    the maximum amount of memory ZRAM can use to store
 146                         the compressed data
 147 num_migrated      RO    the number of objects migrated migrated by compaction
 148 compact           WO    trigger memory compaction
 149
 150 WARNING
 151 =======
 152 per-stat sysfs attributes are considered to be deprecated.
 153 The basic strategy is:
 154 -- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
 155 -- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
 156
 157 The list of deprecated attributes can be found here:
 158 Documentation/ABI/obsolete/sysfs-block-zram
 159
 160 Basically, every attribute that has its own read accessible sysfs node
 161 (e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
 162 or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
 163
 164 User space is advised to use the following files to read the device statistics.
 165
 166 File /sys/block/zram<id>/stat
 167
 168 Represents block layer statistics. Read Documentation/block/stat.txt for
 169 details.
 170
 171 File /sys/block/zram<id>/io_stat
 172
 173 The stat file represents device's I/O statistics not accounted by block
 174 layer and, thus, not available in zram<id>/stat file. It consists of a
 175 single line of text and contains the following stats separated by
 176 whitespace:
 177         failed_reads
 178         failed_writes
 179         invalid_io
 180         notify_free
 181
 182 File /sys/block/zram<id>/mm_stat
 183
 184 The stat file represents device's mm statistics. It consists of a single
 185 line of text and contains the following stats separated by whitespace:
 186         orig_data_size
 187         compr_data_size
 188         mem_used_total
 189         mem_limit
 190         mem_used_max
 191         zero_pages
 192         num_migrated
 193
 194 9) Deactivate:
 195         swapoff /dev/zram0
 196         umount /dev/zram1
 197
 198 10) Reset:
 199         Write any positive value to 'reset' sysfs node
 200         echo 1 > /sys/block/zram0/reset
 201         echo 1 > /sys/block/zram1/reset
 202
 203         This frees all the memory allocated for the given device and
 204         resets the disksize to zero. You must set the disksize again
 205         before reusing the device.
 206
 207 Nitin Gupta
 208 ngupta@vflare.org