1 ======================================== 1 ========================================
2 zram: Compressed RAM-based block devices 2 zram: Compressed RAM-based block devices
3 ======================================== 3 ========================================
4 4
5 Introduction 5 Introduction
6 ============ 6 ============
7 7
8 The zram module creates RAM-based block device 8 The zram module creates RAM-based block devices named /dev/zram<id>
9 (<id> = 0, 1, ...). Pages written to these dis 9 (<id> = 0, 1, ...). Pages written to these disks are compressed and stored
10 in memory itself. These disks allow very fast 10 in memory itself. These disks allow very fast I/O and compression provides
11 good amounts of memory savings. Some of the us 11 good amounts of memory savings. Some of the use cases include /tmp storage,
12 use as swap disks, various caches under /var a 12 use as swap disks, various caches under /var and maybe many more. :)
13 13
14 Statistics for individual zram devices are exp 14 Statistics for individual zram devices are exported through sysfs nodes at
15 /sys/block/zram<id>/ 15 /sys/block/zram<id>/
16 16
17 Usage 17 Usage
18 ===== 18 =====
19 19
20 There are several ways to configure and manage 20 There are several ways to configure and manage zram device(-s):
21 21
22 a) using zram and zram_control sysfs attribute 22 a) using zram and zram_control sysfs attributes
23 b) using zramctl utility, provided by util-lin 23 b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
24 24
25 In this document we will describe only 'manual 25 In this document we will describe only 'manual' zram configuration steps,
26 IOW, zram and zram_control sysfs attributes. 26 IOW, zram and zram_control sysfs attributes.
27 27
28 In order to get a better idea about zramctl pl 28 In order to get a better idea about zramctl please consult util-linux
29 documentation, zramctl man-page or `zramctl -- 29 documentation, zramctl man-page or `zramctl --help`. Please be informed
30 that zram maintainers do not develop/maintain 30 that zram maintainers do not develop/maintain util-linux or zramctl, should
31 you have any questions please contact util-lin 31 you have any questions please contact util-linux@vger.kernel.org
32 32
33 Following shows a typical sequence of steps fo 33 Following shows a typical sequence of steps for using zram.
34 34
35 WARNING 35 WARNING
36 ======= 36 =======
37 37
38 For the sake of simplicity we skip error check 38 For the sake of simplicity we skip error checking parts in most of the
39 examples below. However, it is your sole respo 39 examples below. However, it is your sole responsibility to handle errors.
40 40
41 zram sysfs attributes always return negative v 41 zram sysfs attributes always return negative values in case of errors.
42 The list of possible return codes: 42 The list of possible return codes:
43 43
44 ======== ==================================== 44 ======== =============================================================
45 -EBUSY an attempt to modify an attribute th 45 -EBUSY an attempt to modify an attribute that cannot be changed once
46 the device has been initialised. Ple 46 the device has been initialised. Please reset device first.
47 -ENOMEM zram was not able to allocate enough 47 -ENOMEM zram was not able to allocate enough memory to fulfil your
48 needs. 48 needs.
49 -EINVAL invalid input has been provided. 49 -EINVAL invalid input has been provided.
50 ======== ==================================== 50 ======== =============================================================
51 51
52 If you use 'echo', the returned value is set b 52 If you use 'echo', the returned value is set by the 'echo' utility,
53 and, in general case, something like:: 53 and, in general case, something like::
54 54
55 echo 3 > /sys/block/zram0/max_comp_str 55 echo 3 > /sys/block/zram0/max_comp_streams
56 if [ $? -ne 0 ]; then 56 if [ $? -ne 0 ]; then
57 handle_error 57 handle_error
58 fi 58 fi
59 59
60 should suffice. 60 should suffice.
61 61
62 1) Load Module 62 1) Load Module
63 ============== 63 ==============
64 64
65 :: 65 ::
66 66
67 modprobe zram num_devices=4 67 modprobe zram num_devices=4
68 68
69 This creates 4 devices: /dev/zram{0,1,2,3} 69 This creates 4 devices: /dev/zram{0,1,2,3}
70 70
71 num_devices parameter is optional and tells zr 71 num_devices parameter is optional and tells zram how many devices should be
72 pre-created. Default: 1. 72 pre-created. Default: 1.
73 73
74 2) Set max number of compression streams 74 2) Set max number of compression streams
75 ======================================== 75 ========================================
76 76
77 Regardless of the value passed to this attribu 77 Regardless of the value passed to this attribute, ZRAM will always
78 allocate multiple compression streams - one pe 78 allocate multiple compression streams - one per online CPU - thus
79 allowing several concurrent compression operat 79 allowing several concurrent compression operations. The number of
80 allocated compression streams goes down when s 80 allocated compression streams goes down when some of the CPUs
81 become offline. There is no single-compression 81 become offline. There is no single-compression-stream mode anymore,
82 unless you are running a UP system or have onl 82 unless you are running a UP system or have only 1 CPU online.
83 83
84 To find out how many streams are currently ava 84 To find out how many streams are currently available::
85 85
86 cat /sys/block/zram0/max_comp_streams 86 cat /sys/block/zram0/max_comp_streams
87 87
88 3) Select compression algorithm 88 3) Select compression algorithm
89 =============================== 89 ===============================
90 90
91 Using comp_algorithm device attribute one can 91 Using comp_algorithm device attribute one can see available and
92 currently selected (shown in square brackets) 92 currently selected (shown in square brackets) compression algorithms,
93 or change the selected compression algorithm ( 93 or change the selected compression algorithm (once the device is initialised
94 there is no way to change compression algorith 94 there is no way to change compression algorithm).
95 95
96 Examples:: 96 Examples::
97 97
98 #show supported compression algorithms 98 #show supported compression algorithms
99 cat /sys/block/zram0/comp_algorithm 99 cat /sys/block/zram0/comp_algorithm
100 lzo [lz4] 100 lzo [lz4]
101 101
102 #select lzo compression algorithm 102 #select lzo compression algorithm
103 echo lzo > /sys/block/zram0/comp_algor 103 echo lzo > /sys/block/zram0/comp_algorithm
104 104
105 For the time being, the `comp_algorithm` conte 105 For the time being, the `comp_algorithm` content shows only compression
106 algorithms that are supported by zram. 106 algorithms that are supported by zram.
107 107
108 4) Set compression algorithm parameters: Optio 108 4) Set compression algorithm parameters: Optional
109 ============================================== 109 =================================================
110 110
111 Compression algorithms may support specific pa 111 Compression algorithms may support specific parameters which can be
112 tweaked for particular dataset. ZRAM has an `a 112 tweaked for particular dataset. ZRAM has an `algorithm_params` device
113 attribute which provides a per-algorithm param 113 attribute which provides a per-algorithm params configuration.
114 114
115 For example, several compression algorithms su 115 For example, several compression algorithms support `level` parameter.
116 In addition, certain compression algorithms su 116 In addition, certain compression algorithms support pre-trained dictionaries,
117 which significantly change algorithms' charact 117 which significantly change algorithms' characteristics. In order to configure
118 compression algorithm to use external pre-trai 118 compression algorithm to use external pre-trained dictionary, pass full
119 path to the `dict` along with other parameters 119 path to the `dict` along with other parameters::
120 120
121 #pass path to pre-trained zstd diction 121 #pass path to pre-trained zstd dictionary
122 echo "algo=zstd dict=/etc/dictioary" > 122 echo "algo=zstd dict=/etc/dictioary" > /sys/block/zram0/algorithm_params
123 123
124 #same, but using algorithm priority 124 #same, but using algorithm priority
125 echo "priority=1 dict=/etc/dictioary" 125 echo "priority=1 dict=/etc/dictioary" > \
126 /sys/block/zram0/algorithm_par 126 /sys/block/zram0/algorithm_params
127 127
128 #pass path to pre-trained zstd diction 128 #pass path to pre-trained zstd dictionary and compression level
129 echo "algo=zstd level=8 dict=/etc/dict 129 echo "algo=zstd level=8 dict=/etc/dictioary" > \
130 /sys/block/zram0/algorithm_par 130 /sys/block/zram0/algorithm_params
131 131
132 Parameters are algorithm specific: not all alg 132 Parameters are algorithm specific: not all algorithms support pre-trained
133 dictionaries, not all algorithms support `leve 133 dictionaries, not all algorithms support `level`. Furthermore, for certain
134 algorithms `level` controls the compression le 134 algorithms `level` controls the compression level (the higher the value the
135 better the compression ratio, it even can take 135 better the compression ratio, it even can take negatives values for some
136 algorithms), for other algorithms `level` is a 136 algorithms), for other algorithms `level` is acceleration level (the higher
137 the value the lower the compression ratio). 137 the value the lower the compression ratio).
138 138
139 5) Set Disksize 139 5) Set Disksize
140 =============== 140 ===============
141 141
142 Set disk size by writing the value to sysfs no 142 Set disk size by writing the value to sysfs node 'disksize'.
143 The value can be either in bytes or you can us 143 The value can be either in bytes or you can use mem suffixes.
144 Examples:: 144 Examples::
145 145
146 # Initialize /dev/zram0 with 50MB disk 146 # Initialize /dev/zram0 with 50MB disksize
147 echo $((50*1024*1024)) > /sys/block/zr 147 echo $((50*1024*1024)) > /sys/block/zram0/disksize
148 148
149 # Using mem suffixes 149 # Using mem suffixes
150 echo 256K > /sys/block/zram0/disksize 150 echo 256K > /sys/block/zram0/disksize
151 echo 512M > /sys/block/zram0/disksize 151 echo 512M > /sys/block/zram0/disksize
152 echo 1G > /sys/block/zram0/disksize 152 echo 1G > /sys/block/zram0/disksize
153 153
154 Note: 154 Note:
155 There is little point creating a zram of great 155 There is little point creating a zram of greater than twice the size of memory
156 since we expect a 2:1 compression ratio. Note 156 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
157 size of the disk when not in use so a huge zra 157 size of the disk when not in use so a huge zram is wasteful.
158 158
159 6) Set memory limit: Optional 159 6) Set memory limit: Optional
160 ============================= 160 =============================
161 161
162 Set memory limit by writing the value to sysfs 162 Set memory limit by writing the value to sysfs node 'mem_limit'.
163 The value can be either in bytes or you can us 163 The value can be either in bytes or you can use mem suffixes.
164 In addition, you could change the value in run 164 In addition, you could change the value in runtime.
165 Examples:: 165 Examples::
166 166
167 # limit /dev/zram0 with 50MB memory 167 # limit /dev/zram0 with 50MB memory
168 echo $((50*1024*1024)) > /sys/block/zr 168 echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
169 169
170 # Using mem suffixes 170 # Using mem suffixes
171 echo 256K > /sys/block/zram0/mem_limit 171 echo 256K > /sys/block/zram0/mem_limit
172 echo 512M > /sys/block/zram0/mem_limit 172 echo 512M > /sys/block/zram0/mem_limit
173 echo 1G > /sys/block/zram0/mem_limit 173 echo 1G > /sys/block/zram0/mem_limit
174 174
175 # To disable memory limit 175 # To disable memory limit
176 echo 0 > /sys/block/zram0/mem_limit 176 echo 0 > /sys/block/zram0/mem_limit
177 177
178 7) Activate 178 7) Activate
179 =========== 179 ===========
180 180
181 :: 181 ::
182 182
183 mkswap /dev/zram0 183 mkswap /dev/zram0
184 swapon /dev/zram0 184 swapon /dev/zram0
185 185
186 mkfs.ext4 /dev/zram1 186 mkfs.ext4 /dev/zram1
187 mount /dev/zram1 /tmp 187 mount /dev/zram1 /tmp
188 188
189 8) Add/remove zram devices 189 8) Add/remove zram devices
190 ========================== 190 ==========================
191 191
192 zram provides a control interface, which enabl 192 zram provides a control interface, which enables dynamic (on-demand) device
193 addition and removal. 193 addition and removal.
194 194
195 In order to add a new /dev/zramX device, perfo 195 In order to add a new /dev/zramX device, perform a read operation on the hot_add
196 attribute. This will return either the new dev 196 attribute. This will return either the new device's device id (meaning that you
197 can use /dev/zram<id>) or an error code. 197 can use /dev/zram<id>) or an error code.
198 198
199 Example:: 199 Example::
200 200
201 cat /sys/class/zram-control/hot_add 201 cat /sys/class/zram-control/hot_add
202 1 202 1
203 203
204 To remove the existing /dev/zramX device (wher 204 To remove the existing /dev/zramX device (where X is a device id)
205 execute:: 205 execute::
206 206
207 echo X > /sys/class/zram-control/hot_r 207 echo X > /sys/class/zram-control/hot_remove
208 208
209 9) Stats 209 9) Stats
210 ======== 210 ========
211 211
212 Per-device statistics are exported as various 212 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
213 213
214 A brief description of exported device attribu 214 A brief description of exported device attributes follows. For more details
215 please read Documentation/ABI/testing/sysfs-bl 215 please read Documentation/ABI/testing/sysfs-block-zram.
216 216
217 ====================== ====== ============== 217 ====================== ====== ===============================================
218 Name access desc 218 Name access description
219 ====================== ====== ============== 219 ====================== ====== ===============================================
220 disksize RW show and set t 220 disksize RW show and set the device's disk size
221 initstate RO shows the init 221 initstate RO shows the initialization state of the device
222 reset WO trigger device 222 reset WO trigger device reset
223 mem_used_max WO reset the `mem 223 mem_used_max WO reset the `mem_used_max` counter (see later)
224 mem_limit WO specifies the 224 mem_limit WO specifies the maximum amount of memory ZRAM can
225 use to store t 225 use to store the compressed data
226 writeback_limit WO specifies the 226 writeback_limit WO specifies the maximum amount of write IO zram
227 can write out 227 can write out to backing device as 4KB unit
228 writeback_limit_enable RW show and set w 228 writeback_limit_enable RW show and set writeback_limit feature
229 max_comp_streams RW the number of 229 max_comp_streams RW the number of possible concurrent compress
230 operations 230 operations
231 comp_algorithm RW show and chang 231 comp_algorithm RW show and change the compression algorithm
232 algorithm_params WO setup compress 232 algorithm_params WO setup compression algorithm parameters
233 compact WO trigger memory 233 compact WO trigger memory compaction
234 debug_stat RO this file is u 234 debug_stat RO this file is used for zram debugging purposes
235 backing_dev RW set up backend 235 backing_dev RW set up backend storage for zram to write out
236 idle WO mark allocated 236 idle WO mark allocated slot as idle
237 ====================== ====== ============== 237 ====================== ====== ===============================================
238 238
239 239
240 User space is advised to use the following fil 240 User space is advised to use the following files to read the device statistics.
241 241
242 File /sys/block/zram<id>/stat 242 File /sys/block/zram<id>/stat
243 243
244 Represents block layer statistics. Read Docume 244 Represents block layer statistics. Read Documentation/block/stat.rst for
245 details. 245 details.
246 246
247 File /sys/block/zram<id>/io_stat 247 File /sys/block/zram<id>/io_stat
248 248
249 The stat file represents device's I/O statisti 249 The stat file represents device's I/O statistics not accounted by block
250 layer and, thus, not available in zram<id>/sta 250 layer and, thus, not available in zram<id>/stat file. It consists of a
251 single line of text and contains the following 251 single line of text and contains the following stats separated by
252 whitespace: 252 whitespace:
253 253
254 ============= ============================ 254 ============= =============================================================
255 failed_reads The number of failed reads 255 failed_reads The number of failed reads
256 failed_writes The number of failed writes 256 failed_writes The number of failed writes
257 invalid_io The number of non-page-size- 257 invalid_io The number of non-page-size-aligned I/O requests
258 notify_free Depending on device usage sc 258 notify_free Depending on device usage scenario it may account
259 259
260 a) the number of pages freed 260 a) the number of pages freed because of swap slot free
261 notifications 261 notifications
262 b) the number of pages freed 262 b) the number of pages freed because of
263 REQ_OP_DISCARD requests s 263 REQ_OP_DISCARD requests sent by bio. The former ones are
264 sent to a swap block devi 264 sent to a swap block device when a swap slot is freed,
265 which implies that this d 265 which implies that this disk is being used as a swap disk.
266 266
267 The latter ones are sent by 267 The latter ones are sent by filesystem mounted with
268 discard option, whenever som 268 discard option, whenever some data blocks are getting
269 discarded. 269 discarded.
270 ============= ============================ 270 ============= =============================================================
271 271
272 File /sys/block/zram<id>/mm_stat 272 File /sys/block/zram<id>/mm_stat
273 273
274 The mm_stat file represents the device's mm st 274 The mm_stat file represents the device's mm statistics. It consists of a single
275 line of text and contains the following stats 275 line of text and contains the following stats separated by whitespace:
276 276
277 ================ ============================ 277 ================ =============================================================
278 orig_data_size uncompressed size of data st 278 orig_data_size uncompressed size of data stored in this disk.
279 Unit: bytes 279 Unit: bytes
280 compr_data_size compressed size of data stor 280 compr_data_size compressed size of data stored in this disk
281 mem_used_total the amount of memory allocat 281 mem_used_total the amount of memory allocated for this disk. This
282 includes allocator fragmenta 282 includes allocator fragmentation and metadata overhead,
283 allocated for this disk. So, 283 allocated for this disk. So, allocator space efficiency
284 can be calculated using comp 284 can be calculated using compr_data_size and this statistic.
285 Unit: bytes 285 Unit: bytes
286 mem_limit the maximum amount of memory 286 mem_limit the maximum amount of memory ZRAM can use to store
287 the compressed data 287 the compressed data
288 mem_used_max the maximum amount of memory 288 mem_used_max the maximum amount of memory zram has consumed to
289 store the data 289 store the data
290 same_pages the number of same element f 290 same_pages the number of same element filled pages written to this disk.
291 No memory is allocated for s 291 No memory is allocated for such pages.
292 pages_compacted the number of pages freed du 292 pages_compacted the number of pages freed during compaction
293 huge_pages the number of incompressible 293 huge_pages the number of incompressible pages
294 huge_pages_since the number of incompressible 294 huge_pages_since the number of incompressible pages since zram set up
295 ================ ============================ 295 ================ =============================================================
296 296
297 File /sys/block/zram<id>/bd_stat 297 File /sys/block/zram<id>/bd_stat
298 298
299 The bd_stat file represents a device's backing 299 The bd_stat file represents a device's backing device statistics. It consists of
300 a single line of text and contains the followi 300 a single line of text and contains the following stats separated by whitespace:
301 301
302 ============== ============================== 302 ============== =============================================================
303 bd_count size of data written in backin 303 bd_count size of data written in backing device.
304 Unit: 4K bytes 304 Unit: 4K bytes
305 bd_reads the number of reads from backi 305 bd_reads the number of reads from backing device
306 Unit: 4K bytes 306 Unit: 4K bytes
307 bd_writes the number of writes to backin 307 bd_writes the number of writes to backing device
308 Unit: 4K bytes 308 Unit: 4K bytes
309 ============== ============================== 309 ============== =============================================================
310 310
311 10) Deactivate 311 10) Deactivate
312 ============== 312 ==============
313 313
314 :: 314 ::
315 315
316 swapoff /dev/zram0 316 swapoff /dev/zram0
317 umount /dev/zram1 317 umount /dev/zram1
318 318
319 11) Reset 319 11) Reset
320 ========= 320 =========
321 321
322 Write any positive value to 'reset' sy 322 Write any positive value to 'reset' sysfs node::
323 323
324 echo 1 > /sys/block/zram0/rese 324 echo 1 > /sys/block/zram0/reset
325 echo 1 > /sys/block/zram1/rese 325 echo 1 > /sys/block/zram1/reset
326 326
327 This frees all the memory allocated fo 327 This frees all the memory allocated for the given device and
328 resets the disksize to zero. You must 328 resets the disksize to zero. You must set the disksize again
329 before reusing the device. 329 before reusing the device.
330 330
331 Optional Feature 331 Optional Feature
332 ================ 332 ================
333 333
334 writeback 334 writeback
335 --------- 335 ---------
336 336
337 With CONFIG_ZRAM_WRITEBACK, zram can write idl 337 With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
338 to backing storage rather than keeping it in m 338 to backing storage rather than keeping it in memory.
339 To use the feature, admin should set up backin 339 To use the feature, admin should set up backing device via::
340 340
341 echo /dev/sda5 > /sys/block/zramX/back 341 echo /dev/sda5 > /sys/block/zramX/backing_dev
342 342
343 before disksize setting. It supports only part 343 before disksize setting. It supports only partitions at this moment.
344 If admin wants to use incompressible page writ 344 If admin wants to use incompressible page writeback, they could do it via::
345 345
346 echo huge > /sys/block/zramX/writeback 346 echo huge > /sys/block/zramX/writeback
347 347
348 To use idle page writeback, first, user need t 348 To use idle page writeback, first, user need to declare zram pages
349 as idle:: 349 as idle::
350 350
351 echo all > /sys/block/zramX/idle 351 echo all > /sys/block/zramX/idle
352 352
353 From now on, any pages on zram are idle pages. 353 From now on, any pages on zram are idle pages. The idle mark
354 will be removed until someone requests access 354 will be removed until someone requests access of the block.
355 IOW, unless there is access request, those pag 355 IOW, unless there is access request, those pages are still idle pages.
356 Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACT 356 Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled pages can be
357 marked as idle based on how long (in seconds) 357 marked as idle based on how long (in seconds) it's been since they were
358 last accessed:: 358 last accessed::
359 359
360 echo 86400 > /sys/block/zramX/idle 360 echo 86400 > /sys/block/zramX/idle
361 361
362 In this example all pages which haven't been a 362 In this example all pages which haven't been accessed in more than 86400
363 seconds (one day) will be marked idle. 363 seconds (one day) will be marked idle.
364 364
365 Admin can request writeback of those idle page 365 Admin can request writeback of those idle pages at right timing via::
366 366
367 echo idle > /sys/block/zramX/writeback 367 echo idle > /sys/block/zramX/writeback
368 368
369 With the command, zram will writeback idle pag 369 With the command, zram will writeback idle pages from memory to the storage.
370 370
371 Additionally, if a user choose to writeback on 371 Additionally, if a user choose to writeback only huge and idle pages
372 this can be accomplished with:: 372 this can be accomplished with::
373 373
374 echo huge_idle > /sys/block/zramX/writ 374 echo huge_idle > /sys/block/zramX/writeback
375 375
376 If a user chooses to writeback only incompress 376 If a user chooses to writeback only incompressible pages (pages that none of
377 algorithms can compress) this can be accomplis 377 algorithms can compress) this can be accomplished with::
378 378
379 echo incompressible > /sys/block/zramX 379 echo incompressible > /sys/block/zramX/writeback
380 380
381 If an admin wants to write a specific page in 381 If an admin wants to write a specific page in zram device to the backing device,
382 they could write a page index into the interfa 382 they could write a page index into the interface::
383 383
384 echo "page_index=1251" > /sys/block/zr 384 echo "page_index=1251" > /sys/block/zramX/writeback
385 385
386 If there are lots of write IO with flash devic 386 If there are lots of write IO with flash device, potentially, it has
387 flash wearout problem so that admin needs to d 387 flash wearout problem so that admin needs to design write limitation
388 to guarantee storage health for entire product 388 to guarantee storage health for entire product life.
389 389
390 To overcome the concern, zram supports "writeb 390 To overcome the concern, zram supports "writeback_limit" feature.
391 The "writeback_limit_enable"'s default value i 391 The "writeback_limit_enable"'s default value is 0 so that it doesn't limit
392 any writeback. IOW, if admin wants to apply wr 392 any writeback. IOW, if admin wants to apply writeback budget, they should
393 enable writeback_limit_enable via:: 393 enable writeback_limit_enable via::
394 394
395 $ echo 1 > /sys/block/zramX/writeback_ 395 $ echo 1 > /sys/block/zramX/writeback_limit_enable
396 396
397 Once writeback_limit_enable is set, zram doesn 397 Once writeback_limit_enable is set, zram doesn't allow any writeback
398 until admin sets the budget via /sys/block/zra 398 until admin sets the budget via /sys/block/zramX/writeback_limit.
399 399
400 (If admin doesn't enable writeback_limit_enabl 400 (If admin doesn't enable writeback_limit_enable, writeback_limit's value
401 assigned via /sys/block/zramX/writeback_limit 401 assigned via /sys/block/zramX/writeback_limit is meaningless.)
402 402
403 If admin wants to limit writeback as per-day 4 403 If admin wants to limit writeback as per-day 400M, they could do it
404 like below:: 404 like below::
405 405
406 $ MB_SHIFT=20 406 $ MB_SHIFT=20
407 $ 4K_SHIFT=12 407 $ 4K_SHIFT=12
408 $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > 408 $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
409 /sys/block/zram0/writeback_lim 409 /sys/block/zram0/writeback_limit.
410 $ echo 1 > /sys/block/zram0/writeback_ 410 $ echo 1 > /sys/block/zram0/writeback_limit_enable
411 411
412 If admins want to allow further write again on 412 If admins want to allow further write again once the budget is exhausted,
413 they could do it like below:: 413 they could do it like below::
414 414
415 $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > 415 $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
416 /sys/block/zram0/writeback_lim 416 /sys/block/zram0/writeback_limit
417 417
418 If an admin wants to see the remaining writeba 418 If an admin wants to see the remaining writeback budget since last set::
419 419
420 $ cat /sys/block/zramX/writeback_limit 420 $ cat /sys/block/zramX/writeback_limit
421 421
422 If an admin wants to disable writeback limit, 422 If an admin wants to disable writeback limit, they could do::
423 423
424 $ echo 0 > /sys/block/zramX/writeback_ 424 $ echo 0 > /sys/block/zramX/writeback_limit_enable
425 425
426 The writeback_limit count will reset whenever 426 The writeback_limit count will reset whenever you reset zram (e.g.,
427 system reboot, echo 1 > /sys/block/zramX/reset 427 system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of
428 writeback happened until you reset the zram to 428 writeback happened until you reset the zram to allocate extra writeback
429 budget in next setting is user's job. 429 budget in next setting is user's job.
430 430
431 If admin wants to measure writeback count in a 431 If admin wants to measure writeback count in a certain period, they could
432 know it via /sys/block/zram0/bd_stat's 3rd col 432 know it via /sys/block/zram0/bd_stat's 3rd column.
433 433
434 recompression 434 recompression
435 ------------- 435 -------------
436 436
437 With CONFIG_ZRAM_MULTI_COMP, zram can recompre 437 With CONFIG_ZRAM_MULTI_COMP, zram can recompress pages using alternative
438 (secondary) compression algorithms. The basic 438 (secondary) compression algorithms. The basic idea is that alternative
439 compression algorithm can provide better compr 439 compression algorithm can provide better compression ratio at a price of
440 (potentially) slower compression/decompression 440 (potentially) slower compression/decompression speeds. Alternative compression
441 algorithm can, for example, be more successful 441 algorithm can, for example, be more successful compressing huge pages (those
442 that default algorithm failed to compress). An 442 that default algorithm failed to compress). Another application is idle pages
443 recompression - pages that are cold and sit in 443 recompression - pages that are cold and sit in the memory can be recompressed
444 using more effective algorithm and, hence, red 444 using more effective algorithm and, hence, reduce zsmalloc memory usage.
445 445
446 With CONFIG_ZRAM_MULTI_COMP, zram supports up 446 With CONFIG_ZRAM_MULTI_COMP, zram supports up to 4 compression algorithms:
447 one primary and up to 3 secondary ones. Primar 447 one primary and up to 3 secondary ones. Primary zram compressor is explained
448 in "3) Select compression algorithm", secondar 448 in "3) Select compression algorithm", secondary algorithms are configured
449 using recomp_algorithm device attribute. 449 using recomp_algorithm device attribute.
450 450
451 Example::: 451 Example:::
452 452
453 #show supported recompression algorith 453 #show supported recompression algorithms
454 cat /sys/block/zramX/recomp_algorithm 454 cat /sys/block/zramX/recomp_algorithm
455 #1: lzo lzo-rle lz4 lz4hc [zstd] 455 #1: lzo lzo-rle lz4 lz4hc [zstd]
456 #2: lzo lzo-rle lz4 [lz4hc] zstd 456 #2: lzo lzo-rle lz4 [lz4hc] zstd
457 457
458 Alternative compression algorithms are sorted 458 Alternative compression algorithms are sorted by priority. In the example
459 above, zstd is used as the first alternative a 459 above, zstd is used as the first alternative algorithm, which has priority
460 of 1, while lz4hc is configured as a compressi 460 of 1, while lz4hc is configured as a compression algorithm with priority 2.
461 Alternative compression algorithm's priority i 461 Alternative compression algorithm's priority is provided during algorithms
462 configuration::: 462 configuration:::
463 463
464 #select zstd recompression algorithm, 464 #select zstd recompression algorithm, priority 1
465 echo "algo=zstd priority=1" > /sys/blo 465 echo "algo=zstd priority=1" > /sys/block/zramX/recomp_algorithm
466 466
467 #select deflate recompression algorith 467 #select deflate recompression algorithm, priority 2
468 echo "algo=deflate priority=2" > /sys/ 468 echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm
469 469
470 Another device attribute that CONFIG_ZRAM_MULT 470 Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,
471 which controls recompression. 471 which controls recompression.
472 472
473 Examples::: 473 Examples:::
474 474
475 #IDLE pages recompression is activated 475 #IDLE pages recompression is activated by `idle` mode
476 echo "type=idle" > /sys/block/zramX/re 476 echo "type=idle" > /sys/block/zramX/recompress
477 477
478 #HUGE pages recompression is activated 478 #HUGE pages recompression is activated by `huge` mode
479 echo "type=huge" > /sys/block/zram0/re 479 echo "type=huge" > /sys/block/zram0/recompress
480 480
481 #HUGE_IDLE pages recompression is acti 481 #HUGE_IDLE pages recompression is activated by `huge_idle` mode
482 echo "type=huge_idle" > /sys/block/zra 482 echo "type=huge_idle" > /sys/block/zramX/recompress
483 483
484 The number of idle pages can be significant, s 484 The number of idle pages can be significant, so user-space can pass a size
485 threshold (in bytes) to the recompress knob: z 485 threshold (in bytes) to the recompress knob: zram will recompress only pages
486 of equal or greater size::: 486 of equal or greater size:::
487 487
488 #recompress all pages larger than 3000 488 #recompress all pages larger than 3000 bytes
489 echo "threshold=3000" > /sys/block/zra 489 echo "threshold=3000" > /sys/block/zramX/recompress
490 490
491 #recompress idle pages larger than 200 491 #recompress idle pages larger than 2000 bytes
492 echo "type=idle threshold=2000" > /sys 492 echo "type=idle threshold=2000" > /sys/block/zramX/recompress
493 493
494 It is also possible to limit the number of pag 494 It is also possible to limit the number of pages zram re-compression will
495 attempt to recompress::: 495 attempt to recompress:::
496 496
497 echo "type=huge_idle max_pages=42" > / 497 echo "type=huge_idle max_pages=42" > /sys/block/zramX/recompress
498 498
499 Recompression of idle pages requires memory tr 499 Recompression of idle pages requires memory tracking.
500 500
501 During re-compression for every page, that mat 501 During re-compression for every page, that matches re-compression criteria,
502 ZRAM iterates the list of registered alternati 502 ZRAM iterates the list of registered alternative compression algorithms in
503 order of their priorities. ZRAM stops either w 503 order of their priorities. ZRAM stops either when re-compression was
504 successful (re-compressed object is smaller in 504 successful (re-compressed object is smaller in size than the original one)
505 and matches re-compression criteria (e.g. size 505 and matches re-compression criteria (e.g. size threshold) or when there are
506 no secondary algorithms left to try. If none o 506 no secondary algorithms left to try. If none of the secondary algorithms can
507 successfully re-compressed the page such a pag 507 successfully re-compressed the page such a page is marked as incompressible,
508 so ZRAM will not attempt to re-compress it in 508 so ZRAM will not attempt to re-compress it in the future.
509 509
510 This re-compression behaviour, when it iterate 510 This re-compression behaviour, when it iterates through the list of
511 registered compression algorithms, increases o 511 registered compression algorithms, increases our chances of finding the
512 algorithm that successfully compresses a parti 512 algorithm that successfully compresses a particular page. Sometimes, however,
513 it is convenient (and sometimes even necessary 513 it is convenient (and sometimes even necessary) to limit recompression to
514 only one particular algorithm so that it will 514 only one particular algorithm so that it will not try any other algorithms.
515 This can be achieved by providing a `algo` or 515 This can be achieved by providing a `algo` or `priority` parameter:::
516 516
517 #use zstd algorithm only (if registere 517 #use zstd algorithm only (if registered)
518 echo "type=huge algo=zstd" > /sys/bloc 518 echo "type=huge algo=zstd" > /sys/block/zramX/recompress
519 519
520 #use zstd algorithm only (if zstd was 520 #use zstd algorithm only (if zstd was registered under priority 1)
521 echo "type=huge priority=1" > /sys/blo 521 echo "type=huge priority=1" > /sys/block/zramX/recompress
522 522
523 memory tracking 523 memory tracking
524 =============== 524 ===============
525 525
526 With CONFIG_ZRAM_MEMORY_TRACKING, user can kno 526 With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the
527 zram block. It could be useful to catch cold o 527 zram block. It could be useful to catch cold or incompressible
528 pages of the process with*pagemap. 528 pages of the process with*pagemap.
529 529
530 If you enable the feature, you could see block 530 If you enable the feature, you could see block state via
531 /sys/kernel/debug/zram/zram0/block_state". The 531 /sys/kernel/debug/zram/zram0/block_state". The output is as follows::
532 532
533 300 75.033841 .wh... 533 300 75.033841 .wh...
534 301 63.806904 s..... 534 301 63.806904 s.....
535 302 63.806919 ..hi.. 535 302 63.806919 ..hi..
536 303 62.801919 ....r. 536 303 62.801919 ....r.
537 304 146.781902 ..hi.n 537 304 146.781902 ..hi.n
538 538
539 First column 539 First column
540 zram's block index. 540 zram's block index.
541 Second column 541 Second column
542 access time since the system was boote 542 access time since the system was booted
543 Third column 543 Third column
544 state of the block: 544 state of the block:
545 545
546 s: 546 s:
547 same page 547 same page
548 w: 548 w:
549 written page to backing store 549 written page to backing store
550 h: 550 h:
551 huge page 551 huge page
552 i: 552 i:
553 idle page 553 idle page
554 r: 554 r:
555 recompressed page (secondary c 555 recompressed page (secondary compression algorithm)
556 n: 556 n:
557 none (including secondary) of 557 none (including secondary) of algorithms could compress it
558 558
559 First line of above example says 300th block i 559 First line of above example says 300th block is accessed at 75.033841sec
560 and the block's state is huge so it is written 560 and the block's state is huge so it is written back to the backing
561 storage. It's a debugging feature so anyone sh 561 storage. It's a debugging feature so anyone shouldn't rely on it to work
562 properly. 562 properly.
563 563
564 Nitin Gupta 564 Nitin Gupta
565 ngupta@vflare.org 565 ngupta@vflare.org
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