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Documentation / blockdev / zram.txt




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Based on kernel version 3.19. Page generated on 2015-02-13 21:16 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) 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:
94		mkswap /dev/zram0
95		swapon /dev/zram0
96	
97		mkfs.ext4 /dev/zram1
98		mount /dev/zram1 /tmp
99	
100	7) Stats:
101		Per-device statistics are exported as various nodes under
102		/sys/block/zram<id>/
103			disksize
104			num_reads
105			num_writes
106			failed_reads
107			failed_writes
108			invalid_io
109			notify_free
110			zero_pages
111			orig_data_size
112			compr_data_size
113			mem_used_total
114			mem_used_max
115	
116	8) Deactivate:
117		swapoff /dev/zram0
118		umount /dev/zram1
119	
120	9) Reset:
121		Write any positive value to 'reset' sysfs node
122		echo 1 > /sys/block/zram0/reset
123		echo 1 > /sys/block/zram1/reset
124	
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.
128	
129	Nitin Gupta
130	ngupta@vflare.org
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