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Documentation / blockdev / zram.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	zram: Compressed RAM based block devices
2	----------------------------------------
4	* Introduction
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 :)
12	Statistics for individual zram devices are exported through sysfs nodes at
13	/sys/block/zram<id>/
15	* Usage
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).
21	In this document we will describe only 'manual' zram configuration steps,
22	IOW, zram and zram_control sysfs attributes.
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
29	Following shows a typical sequence of steps for using zram.
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.
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.
44	If you use 'echo', the returned value that is changed by 'echo' utility,
45	and, in general case, something like:
47		echo 3 > /sys/block/zram0/max_comp_streams
48		if [ $? -ne 0 ];
49			handle_error
50		fi
52	should suffice.
54	1) Load Module:
55		modprobe zram num_devices=4
56		This creates 4 devices: /dev/zram{0,1,2,3}
58	num_devices parameter is optional and tells zram how many devices should be
59	pre-created. Default: 1.
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.
69	To find out how many streams are currently available:
70		cat /sys/block/zram0/max_comp_streams
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).
78	Examples:
79		#show supported compression algorithms
80		cat /sys/block/zram0/comp_algorithm
81		lzo [lz4]
83		#select lzo compression algorithm
84		echo lzo > /sys/block/zram0/comp_algorithm
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).
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
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
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.
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
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
126		# To disable memory limit
127		echo 0 > /sys/block/zram0/mem_limit
129	6) Activate:
130		mkswap /dev/zram0
131		swapon /dev/zram0
133		mkfs.ext4 /dev/zram1
134		mount /dev/zram1 /tmp
136	7) Add/remove zram devices
138	zram provides a control interface, which enables dynamic (on-demand) device
139	addition and removal.
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.
145	Example:
146		cat /sys/class/zram-control/hot_add
147		1
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
153	8) Stats:
154	Per-device statistics are exported as various nodes under /sys/block/zram<id>/
156	A brief description of exported device attributes. For more details please
157	read Documentation/ABI/testing/sysfs-block-zram.
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	mem_used_max      WO    reset the `mem_used_max' counter (see later)
165	mem_limit         WO    specifies the maximum amount of memory ZRAM can use
166	                        to store the compressed data
167	max_comp_streams  RW    the number of possible concurrent compress operations
168	comp_algorithm    RW    show and change the compression algorithm
169	compact           WO    trigger memory compaction
170	debug_stat        RO    this file is used for zram debugging purposes
171	backing_dev	  RW	set up backend storage for zram to write out
174	User space is advised to use the following files to read the device statistics.
176	File /sys/block/zram<id>/stat
178	Represents block layer statistics. Read Documentation/block/stat.txt for
179	details.
181	File /sys/block/zram<id>/io_stat
183	The stat file represents device's I/O statistics not accounted by block
184	layer and, thus, not available in zram<id>/stat file. It consists of a
185	single line of text and contains the following stats separated by
186	whitespace:
187	 failed_reads     the number of failed reads
188	 failed_writes    the number of failed writes
189	 invalid_io       the number of non-page-size-aligned I/O requests
190	 notify_free      Depending on device usage scenario it may account
191	                  a) the number of pages freed because of swap slot free
192	                  notifications or b) the number of pages freed because of
193	                  REQ_DISCARD requests sent by bio. The former ones are
194	                  sent to a swap block device when a swap slot is freed,
195	                  which implies that this disk is being used as a swap disk.
196	                  The latter ones are sent by filesystem mounted with
197	                  discard option, whenever some data blocks are getting
198	                  discarded.
200	File /sys/block/zram<id>/mm_stat
202	The stat file represents device's mm statistics. It consists of a single
203	line of text and contains the following stats separated by whitespace:
204	 orig_data_size   uncompressed size of data stored in this disk.
205			  This excludes same-element-filled pages (same_pages) since
206			  no memory is allocated for them.
207	                  Unit: bytes
208	 compr_data_size  compressed size of data stored in this disk
209	 mem_used_total   the amount of memory allocated for this disk. This
210	                  includes allocator fragmentation and metadata overhead,
211	                  allocated for this disk. So, allocator space efficiency
212	                  can be calculated using compr_data_size and this statistic.
213	                  Unit: bytes
214	 mem_limit        the maximum amount of memory ZRAM can use to store
215	                  the compressed data
216	 mem_used_max     the maximum amount of memory zram have consumed to
217	                  store the data
218	 same_pages       the number of same element filled pages written to this disk.
219	                  No memory is allocated for such pages.
220	 pages_compacted  the number of pages freed during compaction
222	9) Deactivate:
223		swapoff /dev/zram0
224		umount /dev/zram1
226	10) Reset:
227		Write any positive value to 'reset' sysfs node
228		echo 1 > /sys/block/zram0/reset
229		echo 1 > /sys/block/zram1/reset
231		This frees all the memory allocated for the given device and
232		resets the disksize to zero. You must set the disksize again
233		before reusing the device.
235	* Optional Feature
237	= writeback
239	With incompressible pages, there is no memory saving with zram.
240	Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
241	to backing storage rather than keeping it in memory.
242	User should set up backing device via /sys/block/zramX/backing_dev
243	before disksize setting.
245	Nitin Gupta
246	ngupta@vflare.org
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