Commit | Line | Data |
---|---|---|
00ac9ba0 NG |
1 | zram: Compressed RAM based block devices |
2 | ---------------------------------------- | |
47f9afb3 | 3 | |
47f9afb3 NG |
4 | * Introduction |
5 | ||
9b9913d8 NG |
6 | The zram module creates RAM based block devices named /dev/zram<id> |
7 | (<id> = 0, 1, ...). Pages written to these disks are compressed and stored | |
8 | in memory itself. These disks allow very fast I/O and compression provides | |
9 | good amounts of memory savings. Some of the usecases include /tmp storage, | |
10 | use as swap disks, various caches under /var and maybe many more :) | |
47f9afb3 | 11 | |
9b9913d8 NG |
12 | Statistics for individual zram devices are exported through sysfs nodes at |
13 | /sys/block/zram<id>/ | |
47f9afb3 NG |
14 | |
15 | * Usage | |
16 | ||
00ac9ba0 | 17 | Following shows a typical sequence of steps for using zram. |
47f9afb3 | 18 | |
9b9913d8 | 19 | 1) Load Module: |
00ac9ba0 | 20 | modprobe zram num_devices=4 |
9b9913d8 | 21 | This creates 4 devices: /dev/zram{0,1,2,3} |
47f9afb3 NG |
22 | (num_devices parameter is optional. Default: 1) |
23 | ||
beca3ec7 SS |
24 | 2) 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 | ||
36 | Note: | |
37 | In order to enable compression backend's multi stream support max_comp_streams | |
38 | must be initially set to desired concurrency level before ZRAM device | |
39 | initialisation. Once the device initialised as a single stream compression | |
60a726e3 MK |
40 | backend (max_comp_streams equals to 1), you will see error if you try to change |
41 | the value of max_comp_streams because single stream compression backend | |
42 | implemented as a special case by lock overhead issue and does not support | |
43 | dynamic max_comp_streams. Only multi stream backend supports dynamic | |
44 | max_comp_streams adjustment. | |
beca3ec7 | 45 | |
e46b8a03 SS |
46 | 3) 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 | ||
60 | 4) Set Disksize | |
0231c403 MK |
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 | |
47f9afb3 | 71 | |
e64cd51d SS |
72 | Note: |
73 | There is little point creating a zram of greater than twice the size of memory | |
74 | since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the | |
75 | size of the disk when not in use so a huge zram is wasteful. | |
76 | ||
9ada9da9 MK |
77 | 5) 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 | ||
93 | 6) Activate: | |
00ac9ba0 NG |
94 | mkswap /dev/zram0 |
95 | swapon /dev/zram0 | |
96 | ||
97 | mkfs.ext4 /dev/zram1 | |
98 | mount /dev/zram1 /tmp | |
47f9afb3 | 99 | |
9ada9da9 | 100 | 7) Stats: |
9b9913d8 NG |
101 | Per-device statistics are exported as various nodes under |
102 | /sys/block/zram<id>/ | |
103 | disksize | |
104 | num_reads | |
105 | num_writes | |
8dd1d324 SS |
106 | failed_reads |
107 | failed_writes | |
9b9913d8 NG |
108 | invalid_io |
109 | notify_free | |
9b9913d8 NG |
110 | zero_pages |
111 | orig_data_size | |
112 | compr_data_size | |
113 | mem_used_total | |
461a8eee | 114 | mem_used_max |
47f9afb3 | 115 | |
9ada9da9 | 116 | 8) Deactivate: |
00ac9ba0 NG |
117 | swapoff /dev/zram0 |
118 | umount /dev/zram1 | |
47f9afb3 | 119 | |
9ada9da9 | 120 | 9) Reset: |
9b9913d8 NG |
121 | Write any positive value to 'reset' sysfs node |
122 | echo 1 > /sys/block/zram0/reset | |
123 | echo 1 > /sys/block/zram1/reset | |
124 | ||
0231c403 MK |
125 | This frees all the memory allocated for the given device and |
126 | resets the disksize to zero. You must set the disksize again | |
127 | before reusing the device. | |
47f9afb3 | 128 | |
47f9afb3 NG |
129 | Nitin Gupta |
130 | ngupta@vflare.org |