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 For the time being, the `comp_algorithm' content does not necessarily
  87 show every compression algorithm supported by the kernel. We keep this
  88 list primarily to simplify device configuration and one can configure
  89 a new device with a compression algorithm that is not listed in
  90 `comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API
  91 and, if some of the algorithms were built as modules, it's impossible
  92 to list all of them using, for instance, /proc/crypto or any other
  93 method. This, however, has an advantage of permitting the usage of
  94 custom crypto compression modules (implementing S/W or H/W compression).
  95
  96 4) Set Disksize
  97 Set disk size by writing the value to sysfs node 'disksize'.
  98 The value can be either in bytes or you can use mem suffixes.
  99 Examples:
 100         # Initialize /dev/zram0 with 50MB disksize
 101         echo $((50*1024*1024)) > /sys/block/zram0/disksize
 102
 103         # Using mem suffixes
 104         echo 256K > /sys/block/zram0/disksize
 105         echo 512M > /sys/block/zram0/disksize
 106         echo 1G > /sys/block/zram0/disksize
 107
 108 Note:
 109 There is little point creating a zram of greater than twice the size of memory
 110 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
 111 size of the disk when not in use so a huge zram is wasteful.
 112
 113 5) Set memory limit: Optional
 114 Set memory limit by writing the value to sysfs node 'mem_limit'.
 115 The value can be either in bytes or you can use mem suffixes.
 116 In addition, you could change the value in runtime.
 117 Examples:
 118         # limit /dev/zram0 with 50MB memory
 119         echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
 120
 121         # Using mem suffixes
 122         echo 256K > /sys/block/zram0/mem_limit
 123         echo 512M > /sys/block/zram0/mem_limit
 124         echo 1G > /sys/block/zram0/mem_limit
 125
 126         # To disable memory limit
 127         echo 0 > /sys/block/zram0/mem_limit
 128
 129 6) Activate:
 130         mkswap /dev/zram0
 131         swapon /dev/zram0
 132
 133         mkfs.ext4 /dev/zram1
 134         mount /dev/zram1 /tmp
 135
 136 7) Add/remove zram devices
 137
 138 zram provides a control interface, which enables dynamic (on-demand) device
 139 addition and removal.
 140
 141 In order to add a new /dev/zramX device, perform read operation on hot_add
 142 attribute. This will return either new device's device id (meaning that you
 143 can use /dev/zram<id>) or error code.
 144
 145 Example:
 146         cat /sys/class/zram-control/hot_add
 147         1
 148
 149 To remove the existing /dev/zramX device (where X is a device id)
 150 execute
 151         echo X > /sys/class/zram-control/hot_remove
 152
 153 8) Stats:
 154 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
 155
 156 A brief description of exported device attributes. For more details please
 157 read Documentation/ABI/testing/sysfs-block-zram.
 158
 159 Name            access            description
 160 ----            ------            -----------
 161 disksize          RW    show and set the device's disk size
 162 initstate         RO    shows the initialization state of the device
 163 reset             WO    trigger device reset
 164 mem_used_max      WO    reset the `mem_used_max' counter (see later)
 165 mem_limit         WO    specifies the maximum amount of memory ZRAM can use
 166                         to store the compressed data
 167 max_comp_streams  RW    the number of possible concurrent compress operations
 168 comp_algorithm    RW    show and change the compression algorithm
 169 compact           WO    trigger memory compaction
 170 debug_stat        RO    this file is used for zram debugging purposes
 171 backing_dev       RW    set up backend storage for zram to write out
 172
 173
 174 User space is advised to use the following files to read the device statistics.
 175
 176 File /sys/block/zram<id>/stat
 177
 178 Represents block layer statistics. Read Documentation/block/stat.txt for
 179 details.
 180
 181 File /sys/block/zram<id>/io_stat
 182
 183 The stat file represents device's I/O statistics not accounted by block
 184 layer and, thus, not available in zram<id>/stat file. It consists of a
 185 single line of text and contains the following stats separated by
 186 whitespace:
 187  failed_reads     the number of failed reads
 188  failed_writes    the number of failed writes
 189  invalid_io       the number of non-page-size-aligned I/O requests
 190  notify_free      Depending on device usage scenario it may account
 191                   a) the number of pages freed because of swap slot free
 192                   notifications or b) the number of pages freed because of
 193                   REQ_DISCARD requests sent by bio. The former ones are
 194                   sent to a swap block device when a swap slot is freed,
 195                   which implies that this disk is being used as a swap disk.
 196                   The latter ones are sent by filesystem mounted with
 197                   discard option, whenever some data blocks are getting
 198                   discarded.
 199
 200 File /sys/block/zram<id>/mm_stat
 201
 202 The stat file represents device's mm statistics. It consists of a single
 203 line of text and contains the following stats separated by whitespace:
 204  orig_data_size   uncompressed size of data stored in this disk.
 205                   This excludes same-element-filled pages (same_pages) since
 206                   no memory is allocated for them.
 207                   Unit: bytes
 208  compr_data_size  compressed size of data stored in this disk
 209  mem_used_total   the amount of memory allocated for this disk. This
 210                   includes allocator fragmentation and metadata overhead,
 211                   allocated for this disk. So, allocator space efficiency
 212                   can be calculated using compr_data_size and this statistic.
 213                   Unit: bytes
 214  mem_limit        the maximum amount of memory ZRAM can use to store
 215                   the compressed data
 216  mem_used_max     the maximum amount of memory zram have consumed to
 217                   store the data
 218  same_pages       the number of same element filled pages written to this disk.
 219                   No memory is allocated for such pages.
 220  pages_compacted  the number of pages freed during compaction
 221
 222 9) Deactivate:
 223         swapoff /dev/zram0
 224         umount /dev/zram1
 225
 226 10) Reset:
 227         Write any positive value to 'reset' sysfs node
 228         echo 1 > /sys/block/zram0/reset
 229         echo 1 > /sys/block/zram1/reset
 230
 231         This frees all the memory allocated for the given device and
 232         resets the disksize to zero. You must set the disksize again
 233         before reusing the device.
 234
 235 * Optional Feature
 236
 237 = writeback
 238
 239 With incompressible pages, there is no memory saving with zram.
 240 Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
 241 to backing storage rather than keeping it in memory.
 242 User should set up backing device via /sys/block/zramX/backing_dev
 243 before disksize setting.
 244
 245 Nitin Gupta
 246 ngupta@vflare.org