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 62Regardless the value passed to this attribute, ZRAM will always 63allocate multiple compression streams - one per online CPUs - thus 64allowing several concurrent compression operations. The number of 65allocated compression streams goes down when some of the CPUs 66become offline. There is no single-compression-stream mode anymore, 67unless you are running a UP system or has only 1 CPU online. 68 69To find out how many streams are currently available: 70 cat /sys/block/zram0/max_comp_streams 71 723) Select compression algorithm 73Using comp_algorithm device attribute one can see available and 74currently selected (shown in square brackets) compression algorithms, 75change selected compression algorithm (once the device is initialised 76there is no way to change compression algorithm). 77 78Examples: 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 86For the time being, the `comp_algorithm' content does not necessarily 87show every compression algorithm supported by the kernel. We keep this 88list primarily to simplify device configuration and one can configure 89a new device with a compression algorithm that is not listed in 90`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API 91and, if some of the algorithms were built as modules, it's impossible 92to list all of them using, for instance, /proc/crypto or any other 93method. This, however, has an advantage of permitting the usage of 94custom crypto compression modules (implementing S/W or H/W compression). 95 964) Set Disksize 97Set disk size by writing the value to sysfs node 'disksize'. 98The value can be either in bytes or you can use mem suffixes. 99Examples: 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 108Note: 109There is little point creating a zram of greater than twice the size of memory 110since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the 111size of the disk when not in use so a huge zram is wasteful. 112 1135) Set memory limit: Optional 114Set memory limit by writing the value to sysfs node 'mem_limit'. 115The value can be either in bytes or you can use mem suffixes. 116In addition, you could change the value in runtime. 117Examples: 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 1296) Activate: 130 mkswap /dev/zram0 131 swapon /dev/zram0 132 133 mkfs.ext4 /dev/zram1 134 mount /dev/zram1 /tmp 135 1367) Add/remove zram devices 137 138zram provides a control interface, which enables dynamic (on-demand) device 139addition and removal. 140 141In order to add a new /dev/zramX device, perform read operation on hot_add 142attribute. This will return either new device's device id (meaning that you 143can use /dev/zram<id>) or error code. 144 145Example: 146 cat /sys/class/zram-control/hot_add 147 1 148 149To remove the existing /dev/zramX device (where X is a device id) 150execute 151 echo X > /sys/class/zram-control/hot_remove 152 1538) Stats: 154Per-device statistics are exported as various nodes under /sys/block/zram<id>/ 155 156A brief description of exported device attributes. For more details please 157read Documentation/ABI/testing/sysfs-block-zram. 158 159Name access description 160---- ------ ----------- 161disksize RW show and set the device's disk size 162initstate RO shows the initialization state of the device 163reset WO trigger device reset 164num_reads RO the number of reads 165failed_reads RO the number of failed reads 166num_write RO the number of writes 167failed_writes RO the number of failed writes 168invalid_io RO the number of non-page-size-aligned I/O requests 169max_comp_streams RW the number of possible concurrent compress operations 170comp_algorithm RW show and change the compression algorithm 171notify_free RO the number of notifications to free pages (either 172 slot free notifications or REQ_DISCARD requests) 173zero_pages RO the number of zero filled pages written to this disk 174orig_data_size RO uncompressed size of data stored in this disk 175compr_data_size RO compressed size of data stored in this disk 176mem_used_total RO the amount of memory allocated for this disk 177mem_used_max RW the maximum amount of memory zram have consumed to 178 store the data (to reset this counter to the actual 179 current value, write 1 to this attribute) 180mem_limit RW the maximum amount of memory ZRAM can use to store 181 the compressed data 182pages_compacted RO the number of pages freed during compaction 183 (available only via zram<id>/mm_stat node) 184compact WO trigger memory compaction 185debug_stat RO this file is used for zram debugging purposes 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