1zram: Compressed RAM based block devices 2---------------------------------------- 3 4* Introduction 5 6The zram module creates RAM based block devices named /dev/zram<id> 7(<id> = 0, 1, ...). Pages written to these disks are compressed and stored 8in memory itself. These disks allow very fast I/O and compression provides 9good amounts of memory savings. Some of the usecases include /tmp storage, 10use as swap disks, various caches under /var and maybe many more :) 11 12Statistics for individual zram devices are exported through sysfs nodes at 13/sys/block/zram<id>/ 14 15* Usage 16 17There are several ways to configure and manage zram device(-s): 18a) using zram and zram_control sysfs attributes 19b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org). 20 21In this document we will describe only 'manual' zram configuration steps, 22IOW, zram and zram_control sysfs attributes. 23 24In order to get a better idea about zramctl please consult util-linux 25documentation, zramctl man-page or `zramctl --help'. Please be informed 26that zram maintainers do not develop/maintain util-linux or zramctl, should 27you have any questions please contact util-linux@vger.kernel.org 28 29Following shows a typical sequence of steps for using zram. 30 31WARNING 32======= 33For the sake of simplicity we skip error checking parts in most of the 34examples below. However, it is your sole responsibility to handle errors. 35 36zram sysfs attributes always return negative values in case of errors. 37The list of possible return codes: 38-EBUSY -- an attempt to modify an attribute that cannot be changed once 39the device has been initialised. Please reset device first; 40-ENOMEM -- zram was not able to allocate enough memory to fulfil your 41needs; 42-EINVAL -- invalid input has been provided. 43 44If you use 'echo', the returned value that is changed by 'echo' utility, 45and, 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 52should suffice. 53 541) Load Module: 55 modprobe zram num_devices=4 56 This creates 4 devices: /dev/zram{0,1,2,3} 57 58num_devices parameter is optional and tells zram how many devices should be 59pre-created. Default: 1. 60 612) 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 73Note: 74In order to enable compression backend's multi stream support max_comp_streams 75must be initially set to desired concurrency level before ZRAM device 76initialisation. Once the device initialised as a single stream compression 77backend (max_comp_streams equals to 1), you will see error if you try to change 78the value of max_comp_streams because single stream compression backend 79implemented as a special case by lock overhead issue and does not support 80dynamic max_comp_streams. Only multi stream backend supports dynamic 81max_comp_streams adjustment. 82 833) 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 96 974) 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 109Note: 110There is little point creating a zram of greater than twice the size of memory 111since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the 112size of the disk when not in use so a huge zram is wasteful. 113 1145) 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 1306) Activate: 131 mkswap /dev/zram0 132 swapon /dev/zram0 133 134 mkfs.ext4 /dev/zram1 135 mount /dev/zram1 /tmp 136 1377) Add/remove zram devices 138 139zram provides a control interface, which enables dynamic (on-demand) device 140addition and removal. 141 142In order to add a new /dev/zramX device, perform read operation on hot_add 143attribute. This will return either new device's device id (meaning that you 144can use /dev/zram<id>) or error code. 145 146Example: 147 cat /sys/class/zram-control/hot_add 148 1 149 150To remove the existing /dev/zramX device (where X is a device id) 151execute 152 echo X > /sys/class/zram-control/hot_remove 153 1548) Stats: 155Per-device statistics are exported as various nodes under /sys/block/zram<id>/ 156 157A brief description of exported device attributes. For more details please 158read Documentation/ABI/testing/sysfs-block-zram. 159 160Name access description 161---- ------ ----------- 162disksize RW show and set the device's disk size 163initstate RO shows the initialization state of the device 164reset WO trigger device reset 165num_reads RO the number of reads 166failed_reads RO the number of failed reads 167num_write RO the number of writes 168failed_writes RO the number of failed writes 169invalid_io RO the number of non-page-size-aligned I/O requests 170max_comp_streams RW the number of possible concurrent compress operations 171comp_algorithm RW show and change the compression algorithm 172notify_free RO the number of notifications to free pages (either 173 slot free notifications or REQ_DISCARD requests) 174zero_pages RO the number of zero filled pages written to this disk 175orig_data_size RO uncompressed size of data stored in this disk 176compr_data_size RO compressed size of data stored in this disk 177mem_used_total RO the amount of memory allocated for this disk 178mem_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) 181mem_limit RW the maximum amount of memory ZRAM can use to store 182 the compressed data 183pages_compacted RO the number of pages freed during compaction 184 (available only via zram<id>/mm_stat node) 185compact WO trigger memory compaction 186 187WARNING 188======= 189per-stat sysfs attributes are considered to be deprecated. 190The 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 194The list of deprecated attributes can be found here: 195Documentation/ABI/obsolete/sysfs-block-zram 196 197Basically, 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 199or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated. 200 201User space is advised to use the following files to read the device statistics. 202 203File /sys/block/zram<id>/stat 204 205Represents block layer statistics. Read Documentation/block/stat.txt for 206details. 207 208File /sys/block/zram<id>/io_stat 209 210The stat file represents device's I/O statistics not accounted by block 211layer and, thus, not available in zram<id>/stat file. It consists of a 212single line of text and contains the following stats separated by 213whitespace: 214 failed_reads 215 failed_writes 216 invalid_io 217 notify_free 218 219File /sys/block/zram<id>/mm_stat 220 221The stat file represents device's mm statistics. It consists of a single 222line 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 2319) Deactivate: 232 swapoff /dev/zram0 233 umount /dev/zram1 234 23510) 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 244Nitin Gupta 245ngupta@vflare.org 246