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Documentation / blockdev / zram.txt




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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 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 /sys/block/zram<id>/
102	
103	A brief description of exported device attritbutes. For more details please
104	read Documentation/ABI/testing/sysfs-block-zram.
105	
106	Name            access            description
107	----            ------            -----------
108	disksize          RW    show and set the device's disk size
109	initstate         RO    shows the initialization state of the device
110	reset             WO    trigger device reset
111	num_reads         RO    the number of reads
112	failed_reads      RO    the number of failed reads
113	num_write         RO    the number of writes
114	failed_writes     RO    the number of failed writes
115	invalid_io        RO    the number of non-page-size-aligned I/O requests
116	max_comp_streams  RW    the number of possible concurrent compress operations
117	comp_algorithm    RW    show and change the compression algorithm
118	notify_free       RO    the number of notifications to free pages (either
119	                        slot free notifications or REQ_DISCARD requests)
120	zero_pages        RO    the number of zero filled pages written to this disk
121	orig_data_size    RO    uncompressed size of data stored in this disk
122	compr_data_size   RO    compressed size of data stored in this disk
123	mem_used_total    RO    the amount of memory allocated for this disk
124	mem_used_max      RW    the maximum amount memory zram have consumed to
125	                        store compressed data
126	mem_limit         RW    the maximum amount of memory ZRAM can use to store
127	                        the compressed data
128	num_migrated      RO    the number of objects migrated migrated by compaction
129	
130	
131	WARNING
132	=======
133	per-stat sysfs attributes are considered to be deprecated.
134	The basic strategy is:
135	-- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
136	-- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
137	
138	The list of deprecated attributes can be found here:
139	Documentation/ABI/obsolete/sysfs-block-zram
140	
141	Basically, every attribute that has its own read accessible sysfs node
142	(e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
143	or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
144	
145	User space is advised to use the following files to read the device statistics.
146	
147	File /sys/block/zram<id>/stat
148	
149	Represents block layer statistics. Read Documentation/block/stat.txt for
150	details.
151	
152	File /sys/block/zram<id>/io_stat
153	
154	The stat file represents device's I/O statistics not accounted by block
155	layer and, thus, not available in zram<id>/stat file. It consists of a
156	single line of text and contains the following stats separated by
157	whitespace:
158		failed_reads
159		failed_writes
160		invalid_io
161		notify_free
162	
163	File /sys/block/zram<id>/mm_stat
164	
165	The stat file represents device's mm statistics. It consists of a single
166	line of text and contains the following stats separated by whitespace:
167		orig_data_size
168		compr_data_size
169		mem_used_total
170		mem_limit
171		mem_used_max
172		zero_pages
173		num_migrated
174	
175	8) Deactivate:
176		swapoff /dev/zram0
177		umount /dev/zram1
178	
179	9) Reset:
180		Write any positive value to 'reset' sysfs node
181		echo 1 > /sys/block/zram0/reset
182		echo 1 > /sys/block/zram1/reset
183	
184		This frees all the memory allocated for the given device and
185		resets the disksize to zero. You must set the disksize again
186		before reusing the device.
187	
188	Nitin Gupta
189	ngupta@vflare.org
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