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         Compression backend may use up to max_comp_streams compression streams,
  63         thus allowing up to max_comp_streams concurrent compression operations.
  64         By default, compression backend uses single compression stream.
  65
  66         Examples:
  67         #show max compression streams number
  68         cat /sys/block/zram0/max_comp_streams
  69
  70         #set max compression streams number to 3
  71         echo 3 > /sys/block/zram0/max_comp_streams
  72
  73 Note:
  74 In order to enable compression backend's multi stream support max_comp_streams
  75 must be initially set to desired concurrency level before ZRAM device
  76 initialisation. Once the device initialised as a single stream compression
  77 backend (max_comp_streams equals to 1), you will see error if you try to change
  78 the value of max_comp_streams because single stream compression backend
  79 implemented as a special case by lock overhead issue and does not support
  80 dynamic max_comp_streams. Only multi stream backend supports dynamic
  81 max_comp_streams adjustment.
  82
  83 3) Select compression algorithm
  84         Using comp_algorithm device attribute one can see available and
  85         currently selected (shown in square brackets) compression algorithms,
  86         change selected compression algorithm (once the device is initialised
  87         there is no way to change compression algorithm).
  88
  89         Examples:
  90         #show supported compression algorithms
  91         cat /sys/block/zram0/comp_algorithm
  92         lzo [lz4]
  93
  94         #select lzo compression algorithm
  95         echo lzo > /sys/block/zram0/comp_algorithm
  96
  97 4) Set Disksize
  98         Set disk size by writing the value to sysfs node 'disksize'.
  99         The value can be either in bytes or you can use mem suffixes.
 100         Examples:
 101             # Initialize /dev/zram0 with 50MB disksize
 102             echo $((50*1024*1024)) > /sys/block/zram0/disksize
 103
 104             # Using mem suffixes
 105             echo 256K > /sys/block/zram0/disksize
 106             echo 512M > /sys/block/zram0/disksize
 107             echo 1G > /sys/block/zram0/disksize
 108
 109 Note:
 110 There is little point creating a zram of greater than twice the size of memory
 111 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
 112 size of the disk when not in use so a huge zram is wasteful.
 113
 114 5) Set memory limit: Optional
 115         Set memory limit by writing the value to sysfs node 'mem_limit'.
 116         The value can be either in bytes or you can use mem suffixes.
 117         In addition, you could change the value in runtime.
 118         Examples:
 119             # limit /dev/zram0 with 50MB memory
 120             echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
 121
 122             # Using mem suffixes
 123             echo 256K > /sys/block/zram0/mem_limit
 124             echo 512M > /sys/block/zram0/mem_limit
 125             echo 1G > /sys/block/zram0/mem_limit
 126
 127             # To disable memory limit
 128             echo 0 > /sys/block/zram0/mem_limit
 129
 130 6) Activate:
 131         mkswap /dev/zram0
 132         swapon /dev/zram0
 133
 134         mkfs.ext4 /dev/zram1
 135         mount /dev/zram1 /tmp
 136
 137 7) Add/remove zram devices
 138
 139 zram provides a control interface, which enables dynamic (on-demand) device
 140 addition and removal.
 141
 142 In order to add a new /dev/zramX device, perform read operation on hot_add
 143 attribute. This will return either new device's device id (meaning that you
 144 can use /dev/zram<id>) or error code.
 145
 146 Example:
 147         cat /sys/class/zram-control/hot_add
 148         1
 149
 150 To remove the existing /dev/zramX device (where X is a device id)
 151 execute
 152         echo X > /sys/class/zram-control/hot_remove
 153
 154 8) Stats:
 155 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
 156
 157 A brief description of exported device attributes. For more details please
 158 read Documentation/ABI/testing/sysfs-block-zram.
 159
 160 Name            access            description
 161 ----            ------            -----------
 162 disksize          RW    show and set the device's disk size
 163 initstate         RO    shows the initialization state of the device
 164 reset             WO    trigger device reset
 165 num_reads         RO    the number of reads
 166 failed_reads      RO    the number of failed reads
 167 num_write         RO    the number of writes
 168 failed_writes     RO    the number of failed writes
 169 invalid_io        RO    the number of non-page-size-aligned I/O requests
 170 max_comp_streams  RW    the number of possible concurrent compress operations
 171 comp_algorithm    RW    show and change the compression algorithm
 172 notify_free       RO    the number of notifications to free pages (either
 173                         slot free notifications or REQ_DISCARD requests)
 174 zero_pages        RO    the number of zero filled pages written to this disk
 175 orig_data_size    RO    uncompressed size of data stored in this disk
 176 compr_data_size   RO    compressed size of data stored in this disk
 177 mem_used_total    RO    the amount of memory allocated for this disk
 178 mem_used_max      RW    the maximum amount of memory zram have consumed to
 179                         store the data (to reset this counter to the actual
 180                         current value, write 1 to this attribute)
 181 mem_limit         RW    the maximum amount of memory ZRAM can use to store
 182                         the compressed data
 183 pages_compacted   RO    the number of pages freed during compaction
 184                         (available only via zram<id>/mm_stat node)
 185 compact           WO    trigger memory compaction
 186
 187 WARNING
 188 =======
 189 per-stat sysfs attributes are considered to be deprecated.
 190 The basic strategy is:
 191 -- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
 192 -- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
 193
 194 The list of deprecated attributes can be found here:
 195 Documentation/ABI/obsolete/sysfs-block-zram
 196
 197 Basically, every attribute that has its own read accessible sysfs node
 198 (e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
 199 or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
 200
 201 User space is advised to use the following files to read the device statistics.
 202
 203 File /sys/block/zram<id>/stat
 204
 205 Represents block layer statistics. Read Documentation/block/stat.txt for
 206 details.
 207
 208 File /sys/block/zram<id>/io_stat
 209
 210 The stat file represents device's I/O statistics not accounted by block
 211 layer and, thus, not available in zram<id>/stat file. It consists of a
 212 single line of text and contains the following stats separated by
 213 whitespace:
 214         failed_reads
 215         failed_writes
 216         invalid_io
 217         notify_free
 218
 219 File /sys/block/zram<id>/mm_stat
 220
 221 The stat file represents device's mm statistics. It consists of a single
 222 line of text and contains the following stats separated by whitespace:
 223         orig_data_size
 224         compr_data_size
 225         mem_used_total
 226         mem_limit
 227         mem_used_max
 228         zero_pages
 229         num_migrated
 230
 231 9) Deactivate:
 232         swapoff /dev/zram0
 233         umount /dev/zram1
 234
 235 10) Reset:
 236         Write any positive value to 'reset' sysfs node
 237         echo 1 > /sys/block/zram0/reset
 238         echo 1 > /sys/block/zram1/reset
 239
 240         This frees all the memory allocated for the given device and
 241         resets the disksize to zero. You must set the disksize again
 242         before reusing the device.
 243
 244 Nitin Gupta
 245 ngupta@vflare.org