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Nitin Gupta00ac9ba2010-06-01 13:31:26 +05301zram: Compressed RAM based block devices
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Nitin Gupta47f9afb2009-09-22 10:26:54 +05303
Nitin Gupta47f9afb2009-09-22 10:26:54 +05304* Introduction
5
Nitin Gupta9b9913d2010-08-09 22:56:55 +05306The 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 :)
Nitin Gupta47f9afb2009-09-22 10:26:54 +053011
Nitin Gupta9b9913d2010-08-09 22:56:55 +053012Statistics for individual zram devices are exported through sysfs nodes at
13/sys/block/zram<id>/
Nitin Gupta47f9afb2009-09-22 10:26:54 +053014
15* Usage
16
Nitin Gupta00ac9ba2010-06-01 13:31:26 +053017Following shows a typical sequence of steps for using zram.
Nitin Gupta47f9afb2009-09-22 10:26:54 +053018
Nitin Gupta9b9913d2010-08-09 22:56:55 +0530191) Load Module:
Nitin Gupta00ac9ba2010-06-01 13:31:26 +053020 modprobe zram num_devices=4
Nitin Gupta9b9913d2010-08-09 22:56:55 +053021 This creates 4 devices: /dev/zram{0,1,2,3}
Nitin Gupta47f9afb2009-09-22 10:26:54 +053022 (num_devices parameter is optional. Default: 1)
23
Sergey Senozhatskybeca3ec2014-04-07 15:38:14 -0700242) Set max number of compression streams
25 Compression backend may use up to max_comp_streams compression streams,
26 thus allowing up to max_comp_streams concurrent compression operations.
27 By default, compression backend uses single compression stream.
28
29 Examples:
30 #show max compression streams number
31 cat /sys/block/zram0/max_comp_streams
32
33 #set max compression streams number to 3
34 echo 3 > /sys/block/zram0/max_comp_streams
35
36Note:
37In order to enable compression backend's multi stream support max_comp_streams
38must be initially set to desired concurrency level before ZRAM device
39initialisation. Once the device initialised as a single stream compression
Minchan Kim60a726e2014-04-07 15:38:21 -070040backend (max_comp_streams equals to 1), you will see error if you try to change
41the value of max_comp_streams because single stream compression backend
42implemented as a special case by lock overhead issue and does not support
43dynamic max_comp_streams. Only multi stream backend supports dynamic
44max_comp_streams adjustment.
Sergey Senozhatskybeca3ec2014-04-07 15:38:14 -070045
Sergey Senozhatskye46b8a02014-04-07 15:38:17 -0700463) Select compression algorithm
47 Using comp_algorithm device attribute one can see available and
48 currently selected (shown in square brackets) compression algortithms,
49 change selected compression algorithm (once the device is initialised
50 there is no way to change compression algorithm).
51
52 Examples:
53 #show supported compression algorithms
54 cat /sys/block/zram0/comp_algorithm
55 lzo [lz4]
56
57 #select lzo compression algorithm
58 echo lzo > /sys/block/zram0/comp_algorithm
59
604) Set Disksize
Minchan Kim0231c402013-01-30 11:41:40 +090061 Set disk size by writing the value to sysfs node 'disksize'.
62 The value can be either in bytes or you can use mem suffixes.
63 Examples:
64 # Initialize /dev/zram0 with 50MB disksize
65 echo $((50*1024*1024)) > /sys/block/zram0/disksize
Nitin Gupta47f9afb2009-09-22 10:26:54 +053066
Minchan Kim0231c402013-01-30 11:41:40 +090067 # Using mem suffixes
68 echo 256K > /sys/block/zram0/disksize
69 echo 512M > /sys/block/zram0/disksize
70 echo 1G > /sys/block/zram0/disksize
Nitin Gupta47f9afb2009-09-22 10:26:54 +053071
Sergey Senozhatskye64cd512014-04-07 15:38:07 -070072Note:
73There is little point creating a zram of greater than twice the size of memory
74since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
75size of the disk when not in use so a huge zram is wasteful.
76
Minchan Kim9ada9da2014-10-09 15:29:53 -0700775) Set memory limit: Optional
78 Set memory limit by writing the value to sysfs node 'mem_limit'.
79 The value can be either in bytes or you can use mem suffixes.
80 In addition, you could change the value in runtime.
81 Examples:
82 # limit /dev/zram0 with 50MB memory
83 echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
84
85 # Using mem suffixes
86 echo 256K > /sys/block/zram0/mem_limit
87 echo 512M > /sys/block/zram0/mem_limit
88 echo 1G > /sys/block/zram0/mem_limit
89
90 # To disable memory limit
91 echo 0 > /sys/block/zram0/mem_limit
92
936) Activate:
Nitin Gupta00ac9ba2010-06-01 13:31:26 +053094 mkswap /dev/zram0
95 swapon /dev/zram0
96
97 mkfs.ext4 /dev/zram1
98 mount /dev/zram1 /tmp
Nitin Gupta47f9afb2009-09-22 10:26:54 +053099
Minchan Kim9ada9da2014-10-09 15:29:53 -07001007) Stats:
Sergey Senozhatsky77ba0152015-04-15 16:16:00 -0700101Per-device statistics are exported as various nodes under /sys/block/zram<id>/
102
103A brief description of exported device attritbutes. For more details please
104read Documentation/ABI/testing/sysfs-block-zram.
105
106Name access description
107---- ------ -----------
108disksize RW show and set the device's disk size
109initstate RO shows the initialization state of the device
110reset WO trigger device reset
111num_reads RO the number of reads
112failed_reads RO the number of failed reads
113num_write RO the number of writes
114failed_writes RO the number of failed writes
115invalid_io RO the number of non-page-size-aligned I/O requests
116max_comp_streams RW the number of possible concurrent compress operations
117comp_algorithm RW show and change the compression algorithm
118notify_free RO the number of notifications to free pages (either
119 slot free notifications or REQ_DISCARD requests)
120zero_pages RO the number of zero filled pages written to this disk
121orig_data_size RO uncompressed size of data stored in this disk
122compr_data_size RO compressed size of data stored in this disk
123mem_used_total RO the amount of memory allocated for this disk
124mem_used_max RW the maximum amount memory zram have consumed to
125 store compressed data
126mem_limit RW the maximum amount of memory ZRAM can use to store
127 the compressed data
128num_migrated RO the number of objects migrated migrated by compaction
129
130
Sergey Senozhatsky8f7d2822015-04-15 16:16:09 -0700131WARNING
132=======
133per-stat sysfs attributes are considered to be deprecated.
134The basic strategy is:
135-- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
136-- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
137
138The list of deprecated attributes can be found here:
139Documentation/ABI/obsolete/sysfs-block-zram
140
141Basically, every attribute that has its own read accessible sysfs node
142(e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
143or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
144
145User space is advised to use the following files to read the device statistics.
146
Sergey Senozhatsky77ba0152015-04-15 16:16:00 -0700147File /sys/block/zram<id>/stat
148
149Represents block layer statistics. Read Documentation/block/stat.txt for
150details.
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530151
Sergey Senozhatsky2f6a3be2015-04-15 16:16:03 -0700152File /sys/block/zram<id>/io_stat
153
154The stat file represents device's I/O statistics not accounted by block
155layer and, thus, not available in zram<id>/stat file. It consists of a
156single line of text and contains the following stats separated by
157whitespace:
158 failed_reads
159 failed_writes
160 invalid_io
161 notify_free
162
Sergey Senozhatsky4f2109f2015-04-15 16:16:06 -0700163File /sys/block/zram<id>/mm_stat
164
165The stat file represents device's mm statistics. It consists of a single
166line of text and contains the following stats separated by whitespace:
167 orig_data_size
168 compr_data_size
169 mem_used_total
170 mem_limit
171 mem_used_max
172 zero_pages
173 num_migrated
174
Minchan Kim9ada9da2014-10-09 15:29:53 -07001758) Deactivate:
Nitin Gupta00ac9ba2010-06-01 13:31:26 +0530176 swapoff /dev/zram0
177 umount /dev/zram1
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530178
Minchan Kim9ada9da2014-10-09 15:29:53 -07001799) Reset:
Nitin Gupta9b9913d2010-08-09 22:56:55 +0530180 Write any positive value to 'reset' sysfs node
181 echo 1 > /sys/block/zram0/reset
182 echo 1 > /sys/block/zram1/reset
183
Minchan Kim0231c402013-01-30 11:41:40 +0900184 This frees all the memory allocated for the given device and
185 resets the disksize to zero. You must set the disksize again
186 before reusing the device.
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530187
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530188Nitin Gupta
189ngupta@vflare.org