About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / blockdev / zram.txt




Custom Search

Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1	zram: Compressed RAM based block devices
2	----------------------------------------
3	
4	* Introduction
5	
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 :)
11	
12	Statistics for individual zram devices are exported through sysfs nodes at
13	/sys/block/zram<id>/
14	
15	* Usage
16	
17	Following shows a typical sequence of steps for using zram.
18	
19	1) 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	
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
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.
45	
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
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	
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	
77	5) Activate:
78		mkswap /dev/zram0
79		swapon /dev/zram0
80	
81		mkfs.ext4 /dev/zram1
82		mount /dev/zram1 /tmp
83	
84	6) 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	
99	7) Deactivate:
100		swapoff /dev/zram0
101		umount /dev/zram1
102	
103	8) 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	
112	Nitin Gupta
113	ngupta@vflare.org
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.