Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
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