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 There are several ways to configure and manage zram device(-s):
  18 a) using zram and zram_control sysfs attributes
  19 b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
  20
  21 In this document we will describe only 'manual' zram configuration steps,
  22 IOW, zram and zram_control sysfs attributes.
  23
  24 In order to get a better idea about zramctl please consult util-linux
  25 documentation, zramctl man-page or `zramctl --help'. Please be informed
  26 that zram maintainers do not develop/maintain util-linux or zramctl, should
  27 you have any questions please contact util-linux@vger.kernel.org
  28
  29 Following shows a typical sequence of steps for using zram.
  30
  31 WARNING
  32 =======
  33 For the sake of simplicity we skip error checking parts in most of the
  34 examples below. However, it is your sole responsibility to handle errors.
  35
  36 zram sysfs attributes always return negative values in case of errors.
  37 The list of possible return codes:
  38 -EBUSY  -- an attempt to modify an attribute that cannot be changed once
  39 the device has been initialised. Please reset device first;
  40 -ENOMEM -- zram was not able to allocate enough memory to fulfil your
  41 needs;
  42 -EINVAL -- invalid input has been provided.
  43
  44 If you use 'echo', the returned value that is changed by 'echo' utility,
  45 and, in general case, something like:
  46
  47         echo 3 > /sys/block/zram0/max_comp_streams
  48         if [ $? -ne 0 ];
  49                 handle_error
  50         fi
  51
  52 should suffice.
  53
  54 1) Load Module:
  55         modprobe zram num_devices=4
  56         This creates 4 devices: /dev/zram{0,1,2,3}
  57
  58 num_devices parameter is optional and tells zram how many devices should be
  59 pre-created. Default: 1.
  60
  61 2) Set max number of compression streams
  62         Regardless the value passed to this attribute, ZRAM will always
  63         allocate multiple compression streams - one per online CPUs - thus
  64         allowing several concurrent compression operations. The number of
  65         allocated compression streams goes down when some of the CPUs
  66         become offline. There is no single-compression-stream mode anymore,
  67         unless you are running a UP system or has only 1 CPU online.
  68
  69         To find out how many streams are currently available:
  70         cat /sys/block/zram0/max_comp_streams
  71
  72 3) Select compression algorithm
  73         Using comp_algorithm device attribute one can see available and
  74         currently selected (shown in square brackets) compression algorithms,
  75         change selected compression algorithm (once the device is initialised
  76         there is no way to change compression algorithm).
  77
  78         Examples:
  79         #show supported compression algorithms
  80         cat /sys/block/zram0/comp_algorithm
  81         lzo [lz4]
  82
  83         #select lzo compression algorithm
  84         echo lzo > /sys/block/zram0/comp_algorithm
  85
  86 4) Set Disksize
  87         Set disk size by writing the value to sysfs node 'disksize'.
  88         The value can be either in bytes or you can use mem suffixes.
  89         Examples:
  90             # Initialize /dev/zram0 with 50MB disksize
  91             echo $((50*1024*1024)) > /sys/block/zram0/disksize
  92
  93             # Using mem suffixes
  94             echo 256K > /sys/block/zram0/disksize
  95             echo 512M > /sys/block/zram0/disksize
  96             echo 1G > /sys/block/zram0/disksize
  97
  98 Note:
  99 There is little point creating a zram of greater than twice the size of memory
 100 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
 101 size of the disk when not in use so a huge zram is wasteful.
 102
 103 5) Set memory limit: Optional
 104         Set memory limit by writing the value to sysfs node 'mem_limit'.
 105         The value can be either in bytes or you can use mem suffixes.
 106         In addition, you could change the value in runtime.
 107         Examples:
 108             # limit /dev/zram0 with 50MB memory
 109             echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
 110
 111             # Using mem suffixes
 112             echo 256K > /sys/block/zram0/mem_limit
 113             echo 512M > /sys/block/zram0/mem_limit
 114             echo 1G > /sys/block/zram0/mem_limit
 115
 116             # To disable memory limit
 117             echo 0 > /sys/block/zram0/mem_limit
 118
 119 6) Activate:
 120         mkswap /dev/zram0
 121         swapon /dev/zram0
 122
 123         mkfs.ext4 /dev/zram1
 124         mount /dev/zram1 /tmp
 125
 126 7) Add/remove zram devices
 127
 128 zram provides a control interface, which enables dynamic (on-demand) device
 129 addition and removal.
 130
 131 In order to add a new /dev/zramX device, perform read operation on hot_add
 132 attribute. This will return either new device's device id (meaning that you
 133 can use /dev/zram<id>) or error code.
 134
 135 Example:
 136         cat /sys/class/zram-control/hot_add
 137         1
 138
 139 To remove the existing /dev/zramX device (where X is a device id)
 140 execute
 141         echo X > /sys/class/zram-control/hot_remove
 142
 143 8) Stats:
 144 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
 145
 146 A brief description of exported device attributes. For more details please
 147 read Documentation/ABI/testing/sysfs-block-zram.
 148
 149 Name            access            description
 150 ----            ------            -----------
 151 disksize          RW    show and set the device's disk size
 152 initstate         RO    shows the initialization state of the device
 153 reset             WO    trigger device reset
 154 num_reads         RO    the number of reads
 155 failed_reads      RO    the number of failed reads
 156 num_write         RO    the number of writes
 157 failed_writes     RO    the number of failed writes
 158 invalid_io        RO    the number of non-page-size-aligned I/O requests
 159 max_comp_streams  RW    the number of possible concurrent compress operations
 160 comp_algorithm    RW    show and change the compression algorithm
 161 notify_free       RO    the number of notifications to free pages (either
 162                         slot free notifications or REQ_DISCARD requests)
 163 zero_pages        RO    the number of zero filled pages written to this disk
 164 orig_data_size    RO    uncompressed size of data stored in this disk
 165 compr_data_size   RO    compressed size of data stored in this disk
 166 mem_used_total    RO    the amount of memory allocated for this disk
 167 mem_used_max      RW    the maximum amount of memory zram have consumed to
 168                         store the data (to reset this counter to the actual
 169                         current value, write 1 to this attribute)
 170 mem_limit         RW    the maximum amount of memory ZRAM can use to store
 171                         the compressed data
 172 pages_compacted   RO    the number of pages freed during compaction
 173                         (available only via zram<id>/mm_stat node)
 174 compact           WO    trigger memory compaction
 175 debug_stat        RO    this file is used for zram debugging purposes
 176
 177 WARNING
 178 =======
 179 per-stat sysfs attributes are considered to be deprecated.
 180 The basic strategy is:
 181 -- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
 182 -- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
 183
 184 The list of deprecated attributes can be found here:
 185 Documentation/ABI/obsolete/sysfs-block-zram
 186
 187 Basically, every attribute that has its own read accessible sysfs node
 188 (e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
 189 or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
 190
 191 User space is advised to use the following files to read the device statistics.
 192
 193 File /sys/block/zram<id>/stat
 194
 195 Represents block layer statistics. Read Documentation/block/stat.txt for
 196 details.
 197
 198 File /sys/block/zram<id>/io_stat
 199
 200 The stat file represents device's I/O statistics not accounted by block
 201 layer and, thus, not available in zram<id>/stat file. It consists of a
 202 single line of text and contains the following stats separated by
 203 whitespace:
 204         failed_reads
 205         failed_writes
 206         invalid_io
 207         notify_free
 208
 209 File /sys/block/zram<id>/mm_stat
 210
 211 The stat file represents device's mm statistics. It consists of a single
 212 line of text and contains the following stats separated by whitespace:
 213         orig_data_size
 214         compr_data_size
 215         mem_used_total
 216         mem_limit
 217         mem_used_max
 218         zero_pages
 219         num_migrated
 220
 221 9) Deactivate:
 222         swapoff /dev/zram0
 223         umount /dev/zram1
 224
 225 10) Reset:
 226         Write any positive value to 'reset' sysfs node
 227         echo 1 > /sys/block/zram0/reset
 228         echo 1 > /sys/block/zram1/reset
 229
 230         This frees all the memory allocated for the given device and
 231         resets the disksize to zero. You must set the disksize again
 232         before reusing the device.
 233
 234 Nitin Gupta
 235 ngupta@vflare.org