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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:
Nitin Gupta9b9913d2010-08-09 22:56:55 +0530101 Per-device statistics are exported as various nodes under
102 /sys/block/zram<id>/
103 disksize
104 num_reads
105 num_writes
Sergey Senozhatsky8dd1d322014-04-07 15:38:08 -0700106 failed_reads
107 failed_writes
Nitin Gupta9b9913d2010-08-09 22:56:55 +0530108 invalid_io
109 notify_free
Nitin Gupta9b9913d2010-08-09 22:56:55 +0530110 zero_pages
111 orig_data_size
112 compr_data_size
113 mem_used_total
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530114
Minchan Kim9ada9da2014-10-09 15:29:53 -07001158) Deactivate:
Nitin Gupta00ac9ba2010-06-01 13:31:26 +0530116 swapoff /dev/zram0
117 umount /dev/zram1
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530118
Minchan Kim9ada9da2014-10-09 15:29:53 -07001199) Reset:
Nitin Gupta9b9913d2010-08-09 22:56:55 +0530120 Write any positive value to 'reset' sysfs node
121 echo 1 > /sys/block/zram0/reset
122 echo 1 > /sys/block/zram1/reset
123
Minchan Kim0231c402013-01-30 11:41:40 +0900124 This frees all the memory allocated for the given device and
125 resets the disksize to zero. You must set the disksize again
126 before reusing the device.
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530127
Nitin Gupta47f9afb2009-09-22 10:26:54 +0530128Nitin Gupta
129ngupta@vflare.org