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Documentation / blockdev / zram.txt




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Based on kernel version 4.3. Page generated on 2015-11-02 12:44 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	
23	num_devices parameter is optional and tells zram how many devices should be
24	pre-created. Default: 1.
25	
26	2) Set max number of compression streams
27		Compression backend may use up to max_comp_streams compression streams,
28		thus allowing up to max_comp_streams concurrent compression operations.
29		By default, compression backend uses single compression stream.
30	
31		Examples:
32		#show max compression streams number
33		cat /sys/block/zram0/max_comp_streams
34	
35		#set max compression streams number to 3
36		echo 3 > /sys/block/zram0/max_comp_streams
37	
38	Note:
39	In order to enable compression backend's multi stream support max_comp_streams
40	must be initially set to desired concurrency level before ZRAM device
41	initialisation. Once the device initialised as a single stream compression
42	backend (max_comp_streams equals to 1), you will see error if you try to change
43	the value of max_comp_streams because single stream compression backend
44	implemented as a special case by lock overhead issue and does not support
45	dynamic max_comp_streams. Only multi stream backend supports dynamic
46	max_comp_streams adjustment.
47	
48	3) Select compression algorithm
49		Using comp_algorithm device attribute one can see available and
50		currently selected (shown in square brackets) compression algortithms,
51		change selected compression algorithm (once the device is initialised
52		there is no way to change compression algorithm).
53	
54		Examples:
55		#show supported compression algorithms
56		cat /sys/block/zram0/comp_algorithm
57		lzo [lz4]
58	
59		#select lzo compression algorithm
60		echo lzo > /sys/block/zram0/comp_algorithm
61	
62	4) Set Disksize
63	        Set disk size by writing the value to sysfs node 'disksize'.
64	        The value can be either in bytes or you can use mem suffixes.
65	        Examples:
66	            # Initialize /dev/zram0 with 50MB disksize
67	            echo $((50*1024*1024)) > /sys/block/zram0/disksize
68	
69	            # Using mem suffixes
70	            echo 256K > /sys/block/zram0/disksize
71	            echo 512M > /sys/block/zram0/disksize
72	            echo 1G > /sys/block/zram0/disksize
73	
74	Note:
75	There is little point creating a zram of greater than twice the size of memory
76	since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
77	size of the disk when not in use so a huge zram is wasteful.
78	
79	5) Set memory limit: Optional
80		Set memory limit by writing the value to sysfs node 'mem_limit'.
81		The value can be either in bytes or you can use mem suffixes.
82		In addition, you could change the value in runtime.
83		Examples:
84		    # limit /dev/zram0 with 50MB memory
85		    echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
86	
87		    # Using mem suffixes
88		    echo 256K > /sys/block/zram0/mem_limit
89		    echo 512M > /sys/block/zram0/mem_limit
90		    echo 1G > /sys/block/zram0/mem_limit
91	
92		    # To disable memory limit
93		    echo 0 > /sys/block/zram0/mem_limit
94	
95	6) Activate:
96		mkswap /dev/zram0
97		swapon /dev/zram0
98	
99		mkfs.ext4 /dev/zram1
100		mount /dev/zram1 /tmp
101	
102	7) Add/remove zram devices
103	
104	zram provides a control interface, which enables dynamic (on-demand) device
105	addition and removal.
106	
107	In order to add a new /dev/zramX device, perform read operation on hot_add
108	attribute. This will return either new device's device id (meaning that you
109	can use /dev/zram<id>) or error code.
110	
111	Example:
112		cat /sys/class/zram-control/hot_add
113		1
114	
115	To remove the existing /dev/zramX device (where X is a device id)
116	execute
117		echo X > /sys/class/zram-control/hot_remove
118	
119	8) Stats:
120	Per-device statistics are exported as various nodes under /sys/block/zram<id>/
121	
122	A brief description of exported device attritbutes. For more details please
123	read Documentation/ABI/testing/sysfs-block-zram.
124	
125	Name            access            description
126	----            ------            -----------
127	disksize          RW    show and set the device's disk size
128	initstate         RO    shows the initialization state of the device
129	reset             WO    trigger device reset
130	num_reads         RO    the number of reads
131	failed_reads      RO    the number of failed reads
132	num_write         RO    the number of writes
133	failed_writes     RO    the number of failed writes
134	invalid_io        RO    the number of non-page-size-aligned I/O requests
135	max_comp_streams  RW    the number of possible concurrent compress operations
136	comp_algorithm    RW    show and change the compression algorithm
137	notify_free       RO    the number of notifications to free pages (either
138	                        slot free notifications or REQ_DISCARD requests)
139	zero_pages        RO    the number of zero filled pages written to this disk
140	orig_data_size    RO    uncompressed size of data stored in this disk
141	compr_data_size   RO    compressed size of data stored in this disk
142	mem_used_total    RO    the amount of memory allocated for this disk
143	mem_used_max      RW    the maximum amount memory zram have consumed to
144	                        store compressed data
145	mem_limit         RW    the maximum amount of memory ZRAM can use to store
146	                        the compressed data
147	pages_compacted   RO    the number of pages freed during compaction
148	                        (available only via zram<id>/mm_stat node)
149	compact           WO    trigger memory compaction
150	
151	WARNING
152	=======
153	per-stat sysfs attributes are considered to be deprecated.
154	The basic strategy is:
155	-- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
156	-- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
157	
158	The list of deprecated attributes can be found here:
159	Documentation/ABI/obsolete/sysfs-block-zram
160	
161	Basically, every attribute that has its own read accessible sysfs node
162	(e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
163	or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
164	
165	User space is advised to use the following files to read the device statistics.
166	
167	File /sys/block/zram<id>/stat
168	
169	Represents block layer statistics. Read Documentation/block/stat.txt for
170	details.
171	
172	File /sys/block/zram<id>/io_stat
173	
174	The stat file represents device's I/O statistics not accounted by block
175	layer and, thus, not available in zram<id>/stat file. It consists of a
176	single line of text and contains the following stats separated by
177	whitespace:
178		failed_reads
179		failed_writes
180		invalid_io
181		notify_free
182	
183	File /sys/block/zram<id>/mm_stat
184	
185	The stat file represents device's mm statistics. It consists of a single
186	line of text and contains the following stats separated by whitespace:
187		orig_data_size
188		compr_data_size
189		mem_used_total
190		mem_limit
191		mem_used_max
192		zero_pages
193		num_migrated
194	
195	9) Deactivate:
196		swapoff /dev/zram0
197		umount /dev/zram1
198	
199	10) Reset:
200		Write any positive value to 'reset' sysfs node
201		echo 1 > /sys/block/zram0/reset
202		echo 1 > /sys/block/zram1/reset
203	
204		This frees all the memory allocated for the given device and
205		resets the disksize to zero. You must set the disksize again
206		before reusing the device.
207	
208	Nitin Gupta
209	ngupta@vflare.org
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