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Documentation / blockdev / zram.txt




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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:39 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	There are several ways to configure and manage zram device(-s):
18	a) using zram and zram_control sysfs attributes
19	b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
20	
21	In this document we will describe only 'manual' zram configuration steps,
22	IOW, zram and zram_control sysfs attributes.
23	
24	In order to get a better idea about zramctl please consult util-linux
25	documentation, zramctl man-page or `zramctl --help'. Please be informed
26	that zram maintainers do not develop/maintain util-linux or zramctl, should
27	you have any questions please contact util-linux@vger.kernel.org
28	
29	Following shows a typical sequence of steps for using zram.
30	
31	WARNING
32	=======
33	For the sake of simplicity we skip error checking parts in most of the
34	examples below. However, it is your sole responsibility to handle errors.
35	
36	zram sysfs attributes always return negative values in case of errors.
37	The list of possible return codes:
38	-EBUSY	-- an attempt to modify an attribute that cannot be changed once
39	the device has been initialised. Please reset device first;
40	-ENOMEM	-- zram was not able to allocate enough memory to fulfil your
41	needs;
42	-EINVAL	-- invalid input has been provided.
43	
44	If you use 'echo', the returned value that is changed by 'echo' utility,
45	and, in general case, something like:
46	
47		echo 3 > /sys/block/zram0/max_comp_streams
48		if [ $? -ne 0 ];
49			handle_error
50		fi
51	
52	should suffice.
53	
54	1) Load Module:
55		modprobe zram num_devices=4
56		This creates 4 devices: /dev/zram{0,1,2,3}
57	
58	num_devices parameter is optional and tells zram how many devices should be
59	pre-created. Default: 1.
60	
61	2) Set max number of compression streams
62		Regardless the value passed to this attribute, ZRAM will always
63		allocate multiple compression streams - one per online CPUs - thus
64		allowing several concurrent compression operations. The number of
65		allocated compression streams goes down when some of the CPUs
66		become offline. There is no single-compression-stream mode anymore,
67		unless you are running a UP system or has only 1 CPU online.
68	
69		To find out how many streams are currently available:
70		cat /sys/block/zram0/max_comp_streams
71	
72	3) Select compression algorithm
73		Using comp_algorithm device attribute one can see available and
74		currently selected (shown in square brackets) compression algorithms,
75		change selected compression algorithm (once the device is initialised
76		there is no way to change compression algorithm).
77	
78		Examples:
79		#show supported compression algorithms
80		cat /sys/block/zram0/comp_algorithm
81		lzo [lz4]
82	
83		#select lzo compression algorithm
84		echo lzo > /sys/block/zram0/comp_algorithm
85	
86	4) Set Disksize
87	        Set disk size by writing the value to sysfs node 'disksize'.
88	        The value can be either in bytes or you can use mem suffixes.
89	        Examples:
90	            # Initialize /dev/zram0 with 50MB disksize
91	            echo $((50*1024*1024)) > /sys/block/zram0/disksize
92	
93	            # Using mem suffixes
94	            echo 256K > /sys/block/zram0/disksize
95	            echo 512M > /sys/block/zram0/disksize
96	            echo 1G > /sys/block/zram0/disksize
97	
98	Note:
99	There is little point creating a zram of greater than twice the size of memory
100	since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
101	size of the disk when not in use so a huge zram is wasteful.
102	
103	5) Set memory limit: Optional
104		Set memory limit by writing the value to sysfs node 'mem_limit'.
105		The value can be either in bytes or you can use mem suffixes.
106		In addition, you could change the value in runtime.
107		Examples:
108		    # limit /dev/zram0 with 50MB memory
109		    echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
110	
111		    # Using mem suffixes
112		    echo 256K > /sys/block/zram0/mem_limit
113		    echo 512M > /sys/block/zram0/mem_limit
114		    echo 1G > /sys/block/zram0/mem_limit
115	
116		    # To disable memory limit
117		    echo 0 > /sys/block/zram0/mem_limit
118	
119	6) Activate:
120		mkswap /dev/zram0
121		swapon /dev/zram0
122	
123		mkfs.ext4 /dev/zram1
124		mount /dev/zram1 /tmp
125	
126	7) Add/remove zram devices
127	
128	zram provides a control interface, which enables dynamic (on-demand) device
129	addition and removal.
130	
131	In order to add a new /dev/zramX device, perform read operation on hot_add
132	attribute. This will return either new device's device id (meaning that you
133	can use /dev/zram<id>) or error code.
134	
135	Example:
136		cat /sys/class/zram-control/hot_add
137		1
138	
139	To remove the existing /dev/zramX device (where X is a device id)
140	execute
141		echo X > /sys/class/zram-control/hot_remove
142	
143	8) Stats:
144	Per-device statistics are exported as various nodes under /sys/block/zram<id>/
145	
146	A brief description of exported device attributes. For more details please
147	read Documentation/ABI/testing/sysfs-block-zram.
148	
149	Name            access            description
150	----            ------            -----------
151	disksize          RW    show and set the device's disk size
152	initstate         RO    shows the initialization state of the device
153	reset             WO    trigger device reset
154	num_reads         RO    the number of reads
155	failed_reads      RO    the number of failed reads
156	num_write         RO    the number of writes
157	failed_writes     RO    the number of failed writes
158	invalid_io        RO    the number of non-page-size-aligned I/O requests
159	max_comp_streams  RW    the number of possible concurrent compress operations
160	comp_algorithm    RW    show and change the compression algorithm
161	notify_free       RO    the number of notifications to free pages (either
162	                        slot free notifications or REQ_DISCARD requests)
163	zero_pages        RO    the number of zero filled pages written to this disk
164	orig_data_size    RO    uncompressed size of data stored in this disk
165	compr_data_size   RO    compressed size of data stored in this disk
166	mem_used_total    RO    the amount of memory allocated for this disk
167	mem_used_max      RW    the maximum amount of memory zram have consumed to
168	                        store the data (to reset this counter to the actual
169	                        current value, write 1 to this attribute)
170	mem_limit         RW    the maximum amount of memory ZRAM can use to store
171	                        the compressed data
172	pages_compacted   RO    the number of pages freed during compaction
173	                        (available only via zram<id>/mm_stat node)
174	compact           WO    trigger memory compaction
175	debug_stat        RO    this file is used for zram debugging purposes
176	
177	WARNING
178	=======
179	per-stat sysfs attributes are considered to be deprecated.
180	The basic strategy is:
181	-- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
182	-- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
183	
184	The list of deprecated attributes can be found here:
185	Documentation/ABI/obsolete/sysfs-block-zram
186	
187	Basically, every attribute that has its own read accessible sysfs node
188	(e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
189	or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
190	
191	User space is advised to use the following files to read the device statistics.
192	
193	File /sys/block/zram<id>/stat
194	
195	Represents block layer statistics. Read Documentation/block/stat.txt for
196	details.
197	
198	File /sys/block/zram<id>/io_stat
199	
200	The stat file represents device's I/O statistics not accounted by block
201	layer and, thus, not available in zram<id>/stat file. It consists of a
202	single line of text and contains the following stats separated by
203	whitespace:
204		failed_reads
205		failed_writes
206		invalid_io
207		notify_free
208	
209	File /sys/block/zram<id>/mm_stat
210	
211	The stat file represents device's mm statistics. It consists of a single
212	line of text and contains the following stats separated by whitespace:
213		orig_data_size
214		compr_data_size
215		mem_used_total
216		mem_limit
217		mem_used_max
218		zero_pages
219		num_migrated
220	
221	9) Deactivate:
222		swapoff /dev/zram0
223		umount /dev/zram1
224	
225	10) Reset:
226		Write any positive value to 'reset' sysfs node
227		echo 1 > /sys/block/zram0/reset
228		echo 1 > /sys/block/zram1/reset
229	
230		This frees all the memory allocated for the given device and
231		resets the disksize to zero. You must set the disksize again
232		before reusing the device.
233	
234	Nitin Gupta
235	ngupta@vflare.org
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