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linux
1zram: Compressed RAM based block devices
2----------------------------------------
3
4* Introduction
5
6The 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 :)
11
12Statistics for individual zram devices are exported through sysfs nodes at
13/sys/block/zram<id>/
14
15* Usage
16
17Following shows a typical sequence of steps for using zram.
18
191) Load Module:
20 modprobe zram num_devices=4
21 This creates 4 devices: /dev/zram{0,1,2,3}
22 (num_devices parameter is optional. Default: 1)
23
242) 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
40backend (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.
45
463) 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
61 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
66
67 # Using mem suffixes
68 echo 256K > /sys/block/zram0/disksize
69 echo 512M > /sys/block/zram0/disksize
70 echo 1G > /sys/block/zram0/disksize
71
72Note:
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
775) Activate:
78 mkswap /dev/zram0
79 swapon /dev/zram0
80
81 mkfs.ext4 /dev/zram1
82 mount /dev/zram1 /tmp
83
846) Stats:
85 Per-device statistics are exported as various nodes under
86 /sys/block/zram<id>/
87 disksize
88 num_reads
89 num_writes
90 failed_reads
91 failed_writes
92 invalid_io
93 notify_free
94 zero_pages
95 orig_data_size
96 compr_data_size
97 mem_used_total
98
997) Deactivate:
100 swapoff /dev/zram0
101 umount /dev/zram1
102
1038) Reset:
104 Write any positive value to 'reset' sysfs node
105 echo 1 > /sys/block/zram0/reset
106 echo 1 > /sys/block/zram1/reset
107
108 This frees all the memory allocated for the given device and
109 resets the disksize to zero. You must set the disksize again
110 before reusing the device.
111
112Nitin Gupta
113ngupta@vflare.org