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