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Documentation / blockdev / zram.txt




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Based on kernel version 4.9. Page generated on 2016-12-21 14:28 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	For the time being, the `comp_algorithm' content does not necessarily
87	show every compression algorithm supported by the kernel. We keep this
88	list primarily to simplify device configuration and one can configure
89	a new device with a compression algorithm that is not listed in
90	`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API
91	and, if some of the algorithms were built as modules, it's impossible
92	to list all of them using, for instance, /proc/crypto or any other
93	method. This, however, has an advantage of permitting the usage of
94	custom crypto compression modules (implementing S/W or H/W compression).
95	
96	4) Set Disksize
97	Set disk size by writing the value to sysfs node 'disksize'.
98	The value can be either in bytes or you can use mem suffixes.
99	Examples:
100		# Initialize /dev/zram0 with 50MB disksize
101		echo $((50*1024*1024)) > /sys/block/zram0/disksize
102	
103		# Using mem suffixes
104		echo 256K > /sys/block/zram0/disksize
105		echo 512M > /sys/block/zram0/disksize
106		echo 1G > /sys/block/zram0/disksize
107	
108	Note:
109	There is little point creating a zram of greater than twice the size of memory
110	since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
111	size of the disk when not in use so a huge zram is wasteful.
112	
113	5) Set memory limit: Optional
114	Set memory limit by writing the value to sysfs node 'mem_limit'.
115	The value can be either in bytes or you can use mem suffixes.
116	In addition, you could change the value in runtime.
117	Examples:
118		# limit /dev/zram0 with 50MB memory
119		echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
120	
121		# Using mem suffixes
122		echo 256K > /sys/block/zram0/mem_limit
123		echo 512M > /sys/block/zram0/mem_limit
124		echo 1G > /sys/block/zram0/mem_limit
125	
126		# To disable memory limit
127		echo 0 > /sys/block/zram0/mem_limit
128	
129	6) Activate:
130		mkswap /dev/zram0
131		swapon /dev/zram0
132	
133		mkfs.ext4 /dev/zram1
134		mount /dev/zram1 /tmp
135	
136	7) Add/remove zram devices
137	
138	zram provides a control interface, which enables dynamic (on-demand) device
139	addition and removal.
140	
141	In order to add a new /dev/zramX device, perform read operation on hot_add
142	attribute. This will return either new device's device id (meaning that you
143	can use /dev/zram<id>) or error code.
144	
145	Example:
146		cat /sys/class/zram-control/hot_add
147		1
148	
149	To remove the existing /dev/zramX device (where X is a device id)
150	execute
151		echo X > /sys/class/zram-control/hot_remove
152	
153	8) Stats:
154	Per-device statistics are exported as various nodes under /sys/block/zram<id>/
155	
156	A brief description of exported device attributes. For more details please
157	read Documentation/ABI/testing/sysfs-block-zram.
158	
159	Name            access            description
160	----            ------            -----------
161	disksize          RW    show and set the device's disk size
162	initstate         RO    shows the initialization state of the device
163	reset             WO    trigger device reset
164	num_reads         RO    the number of reads
165	failed_reads      RO    the number of failed reads
166	num_write         RO    the number of writes
167	failed_writes     RO    the number of failed writes
168	invalid_io        RO    the number of non-page-size-aligned I/O requests
169	max_comp_streams  RW    the number of possible concurrent compress operations
170	comp_algorithm    RW    show and change the compression algorithm
171	notify_free       RO    the number of notifications to free pages (either
172	                        slot free notifications or REQ_DISCARD requests)
173	zero_pages        RO    the number of zero filled pages written to this disk
174	orig_data_size    RO    uncompressed size of data stored in this disk
175	compr_data_size   RO    compressed size of data stored in this disk
176	mem_used_total    RO    the amount of memory allocated for this disk
177	mem_used_max      RW    the maximum amount of memory zram have consumed to
178	                        store the data (to reset this counter to the actual
179	                        current value, write 1 to this attribute)
180	mem_limit         RW    the maximum amount of memory ZRAM can use to store
181	                        the compressed data
182	pages_compacted   RO    the number of pages freed during compaction
183	                        (available only via zram<id>/mm_stat node)
184	compact           WO    trigger memory compaction
185	debug_stat        RO    this file is used for zram debugging purposes
186	
187	WARNING
188	=======
189	per-stat sysfs attributes are considered to be deprecated.
190	The basic strategy is:
191	-- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
192	-- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
193	
194	The list of deprecated attributes can be found here:
195	Documentation/ABI/obsolete/sysfs-block-zram
196	
197	Basically, every attribute that has its own read accessible sysfs node
198	(e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
199	or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
200	
201	User space is advised to use the following files to read the device statistics.
202	
203	File /sys/block/zram<id>/stat
204	
205	Represents block layer statistics. Read Documentation/block/stat.txt for
206	details.
207	
208	File /sys/block/zram<id>/io_stat
209	
210	The stat file represents device's I/O statistics not accounted by block
211	layer and, thus, not available in zram<id>/stat file. It consists of a
212	single line of text and contains the following stats separated by
213	whitespace:
214		failed_reads
215		failed_writes
216		invalid_io
217		notify_free
218	
219	File /sys/block/zram<id>/mm_stat
220	
221	The stat file represents device's mm statistics. It consists of a single
222	line of text and contains the following stats separated by whitespace:
223		orig_data_size
224		compr_data_size
225		mem_used_total
226		mem_limit
227		mem_used_max
228		zero_pages
229		num_migrated
230	
231	9) Deactivate:
232		swapoff /dev/zram0
233		umount /dev/zram1
234	
235	10) Reset:
236		Write any positive value to 'reset' sysfs node
237		echo 1 > /sys/block/zram0/reset
238		echo 1 > /sys/block/zram1/reset
239	
240		This frees all the memory allocated for the given device and
241		resets the disksize to zero. You must set the disksize again
242		before reusing the device.
243	
244	Nitin Gupta
245	ngupta@vflare.org
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