admin-guide/blockdev/zram.rst

========================================
zram: Compressed RAM-based block devices
========================================

Introduction
============

The zram module creates RAM-based block devices named /dev/zram<id>
(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
in memory itself. These disks allow very fast I/O and compression provides
good amounts of memory savings. Some of the use cases include /tmp storage,
use as swap disks, various caches under /var and maybe many more. :)

Statistics for individual zram devices are exported through sysfs nodes at
/sys/block/zram<id>/

Usage
=====

There are several ways to configure and manage zram device(-s):

a) using zram and zram_control sysfs attributes
b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).

In this document we will describe only 'manual' zram configuration steps,
IOW, zram and zram_control sysfs attributes.

In order to get a better idea about zramctl please consult util-linux
documentation, zramctl man-page or `zramctl --help`. Please be informed
that zram maintainers do not develop/maintain util-linux or zramctl, should
you have any questions please contact util-linux@vger.kernel.org

Following shows a typical sequence of steps for using zram.

WARNING
=======

For the sake of simplicity we skip error checking parts in most of the
examples below. However, it is your sole responsibility to handle errors.

zram sysfs attributes always return negative values in case of errors.
The list of possible return codes:

========  =============================================================
-EBUSY	  an attempt to modify an attribute that cannot be changed once
	  the device has been initialised. Please reset device first.
-ENOMEM	  zram was not able to allocate enough memory to fulfil your
	  needs.
-EINVAL	  invalid input has been provided.
========  =============================================================

If you use 'echo', the returned value is set by the 'echo' utility,
and, in general case, something like::

	echo 3 > /sys/block/zram0/max_comp_streams
	if [ $? -ne 0 ]; then
		handle_error
	fi

should suffice.

1) Load Module
==============

::

	modprobe zram num_devices=4

This creates 4 devices: /dev/zram{0,1,2,3}

num_devices parameter is optional and tells zram how many devices should be
pre-created. Default: 1.

2) Set max number of compression streams
========================================

Regardless of the value passed to this attribute, ZRAM will always
allocate multiple compression streams - one per online CPU - thus
allowing several concurrent compression operations. The number of
allocated compression streams goes down when some of the CPUs
become offline. There is no single-compression-stream mode anymore,
unless you are running a UP system or have only 1 CPU online.

To find out how many streams are currently available::

	cat /sys/block/zram0/max_comp_streams

3) Select compression algorithm
===============================

Using comp_algorithm device attribute one can see available and
currently selected (shown in square brackets) compression algorithms,
or change the selected compression algorithm (once the device is initialised
there is no way to change compression algorithm).

Examples::

	#show supported compression algorithms
	cat /sys/block/zram0/comp_algorithm
	lzo [lz4]

	#select lzo compression algorithm
	echo lzo > /sys/block/zram0/comp_algorithm

For the time being, the `comp_algorithm` content does not necessarily
show every compression algorithm supported by the kernel. We keep this
list primarily to simplify device configuration and one can configure
a new device with a compression algorithm that is not listed in
`comp_algorithm`. The thing is that, internally, ZRAM uses Crypto API
and, if some of the algorithms were built as modules, it's impossible
to list all of them using, for instance, /proc/crypto or any other
method. This, however, has an advantage of permitting the usage of
custom crypto compression modules (implementing S/W or H/W compression).

4) Set Disksize
===============

Set disk size by writing the value to sysfs node 'disksize'.
The value can be either in bytes or you can use mem suffixes.
Examples::

	# Initialize /dev/zram0 with 50MB disksize
	echo $((50*1024*1024)) > /sys/block/zram0/disksize

	# Using mem suffixes
	echo 256K > /sys/block/zram0/disksize
	echo 512M > /sys/block/zram0/disksize
	echo 1G > /sys/block/zram0/disksize

Note:
There is little point creating a zram of greater than twice the size of memory
since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
size of the disk when not in use so a huge zram is wasteful.

5) Set memory limit: Optional
=============================

Set memory limit by writing the value to sysfs node 'mem_limit'.
The value can be either in bytes or you can use mem suffixes.
In addition, you could change the value in runtime.
Examples::

	# limit /dev/zram0 with 50MB memory
	echo $((50*1024*1024)) > /sys/block/zram0/mem_limit

	# Using mem suffixes
	echo 256K > /sys/block/zram0/mem_limit
	echo 512M > /sys/block/zram0/mem_limit
	echo 1G > /sys/block/zram0/mem_limit

	# To disable memory limit
	echo 0 > /sys/block/zram0/mem_limit

6) Activate
===========

::

	mkswap /dev/zram0
	swapon /dev/zram0

	mkfs.ext4 /dev/zram1
	mount /dev/zram1 /tmp

7) Add/remove zram devices
==========================

zram provides a control interface, which enables dynamic (on-demand) device
addition and removal.

In order to add a new /dev/zramX device, perform a read operation on the hot_add
attribute. This will return either the new device's device id (meaning that you
can use /dev/zram<id>) or an error code.

Example::

	cat /sys/class/zram-control/hot_add
	1

To remove the existing /dev/zramX device (where X is a device id)
execute::

	echo X > /sys/class/zram-control/hot_remove

8) Stats
========

Per-device statistics are exported as various nodes under /sys/block/zram<id>/

A brief description of exported device attributes follows. For more details
please read Documentation/ABI/testing/sysfs-block-zram.

======================  ======  ===============================================
Name            	access            description
======================  ======  ===============================================
disksize          	RW	show and set the device's disk size
initstate         	RO	shows the initialization state of the device
reset             	WO	trigger device reset
mem_used_max      	WO	reset the `mem_used_max` counter (see later)
mem_limit         	WO	specifies the maximum amount of memory ZRAM can
				use to store the compressed data
writeback_limit   	WO	specifies the maximum amount of write IO zram
				can write out to backing device as 4KB unit
writeback_limit_enable  RW	show and set writeback_limit feature
max_comp_streams  	RW	the number of possible concurrent compress
				operations
comp_algorithm    	RW	show and change the compression algorithm
compact           	WO	trigger memory compaction
debug_stat        	RO	this file is used for zram debugging purposes
backing_dev	  	RW	set up backend storage for zram to write out
idle		  	WO	mark allocated slot as idle
======================  ======  ===============================================


User space is advised to use the following files to read the device statistics.

File /sys/block/zram<id>/stat

Represents block layer statistics. Read Documentation/block/stat.rst for
details.

File /sys/block/zram<id>/io_stat

The stat file represents device's I/O statistics not accounted by block
layer and, thus, not available in zram<id>/stat file. It consists of a
single line of text and contains the following stats separated by
whitespace:

 =============    =============================================================
 failed_reads     The number of failed reads
 failed_writes    The number of failed writes
 invalid_io       The number of non-page-size-aligned I/O requests
 notify_free      Depending on device usage scenario it may account

                  a) the number of pages freed because of swap slot free
                     notifications
                  b) the number of pages freed because of
                     REQ_OP_DISCARD requests sent by bio. The former ones are
                     sent to a swap block device when a swap slot is freed,
                     which implies that this disk is being used as a swap disk.

                  The latter ones are sent by filesystem mounted with
                  discard option, whenever some data blocks are getting
                  discarded.
 =============    =============================================================

File /sys/block/zram<id>/mm_stat

The mm_stat file represents the device's mm statistics. It consists of a single
line of text and contains the following stats separated by whitespace:

 ================ =============================================================
 orig_data_size   uncompressed size of data stored in this disk.
                  Unit: bytes
 compr_data_size  compressed size of data stored in this disk
 mem_used_total   the amount of memory allocated for this disk. This
                  includes allocator fragmentation and metadata overhead,
                  allocated for this disk. So, allocator space efficiency
                  can be calculated using compr_data_size and this statistic.
                  Unit: bytes
 mem_limit        the maximum amount of memory ZRAM can use to store
                  the compressed data
 mem_used_max     the maximum amount of memory zram has consumed to
                  store the data
 same_pages       the number of same element filled pages written to this disk.
                  No memory is allocated for such pages.
 pages_compacted  the number of pages freed during compaction
 huge_pages	  the number of incompressible pages
 huge_pages_since the number of incompressible pages since zram set up
 ================ =============================================================

File /sys/block/zram<id>/bd_stat

The bd_stat file represents a device's backing device statistics. It consists of
a single line of text and contains the following stats separated by whitespace:

 ============== =============================================================
 bd_count	size of data written in backing device.
		Unit: 4K bytes
 bd_reads	the number of reads from backing device
		Unit: 4K bytes
 bd_writes	the number of writes to backing device
		Unit: 4K bytes
 ============== =============================================================

9) Deactivate
=============

::

	swapoff /dev/zram0
	umount /dev/zram1

10) Reset
=========

	Write any positive value to 'reset' sysfs node::

		echo 1 > /sys/block/zram0/reset
		echo 1 > /sys/block/zram1/reset

	This frees all the memory allocated for the given device and
	resets the disksize to zero. You must set the disksize again
	before reusing the device.

Optional Feature
================

writeback
---------

With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
to backing storage rather than keeping it in memory.
To use the feature, admin should set up backing device via::

	echo /dev/sda5 > /sys/block/zramX/backing_dev

before disksize setting. It supports only partitions at this moment.
If admin wants to use incompressible page writeback, they could do it via::

	echo huge > /sys/block/zramX/writeback

To use idle page writeback, first, user need to declare zram pages
as idle::

	echo all > /sys/block/zramX/idle

From now on, any pages on zram are idle pages. The idle mark
will be removed until someone requests access of the block.
IOW, unless there is access request, those pages are still idle pages.
Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled pages can be
marked as idle based on how long (in seconds) it's been since they were
last accessed::

        echo 86400 > /sys/block/zramX/idle

In this example all pages which haven't been accessed in more than 86400
seconds (one day) will be marked idle.

Admin can request writeback of those idle pages at right timing via::

	echo idle > /sys/block/zramX/writeback

With the command, zram will writeback idle pages from memory to the storage.

Additionally, if a user choose to writeback only huge and idle pages
this can be accomplished with::

        echo huge_idle > /sys/block/zramX/writeback

If a user chooses to writeback only incompressible pages (pages that none of
algorithms can compress) this can be accomplished with::

	echo incompressible > /sys/block/zramX/writeback

If an admin wants to write a specific page in zram device to the backing device,
they could write a page index into the interface::

	echo "page_index=1251" > /sys/block/zramX/writeback

If there are lots of write IO with flash device, potentially, it has
flash wearout problem so that admin needs to design write limitation
to guarantee storage health for entire product life.

To overcome the concern, zram supports "writeback_limit" feature.
The "writeback_limit_enable"'s default value is 0 so that it doesn't limit
any writeback. IOW, if admin wants to apply writeback budget, they should
enable writeback_limit_enable via::

	$ echo 1 > /sys/block/zramX/writeback_limit_enable

Once writeback_limit_enable is set, zram doesn't allow any writeback
until admin sets the budget via /sys/block/zramX/writeback_limit.

(If admin doesn't enable writeback_limit_enable, writeback_limit's value
assigned via /sys/block/zramX/writeback_limit is meaningless.)

If admin wants to limit writeback as per-day 400M, they could do it
like below::

	$ MB_SHIFT=20
	$ 4K_SHIFT=12
	$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
		/sys/block/zram0/writeback_limit.
	$ echo 1 > /sys/block/zram0/writeback_limit_enable

If admins want to allow further write again once the budget is exhausted,
they could do it like below::

	$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
		/sys/block/zram0/writeback_limit

If an admin wants to see the remaining writeback budget since last set::

	$ cat /sys/block/zramX/writeback_limit

If an admin wants to disable writeback limit, they could do::

	$ echo 0 > /sys/block/zramX/writeback_limit_enable

The writeback_limit count will reset whenever you reset zram (e.g.,
system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of
writeback happened until you reset the zram to allocate extra writeback
budget in next setting is user's job.

If admin wants to measure writeback count in a certain period, they could
know it via /sys/block/zram0/bd_stat's 3rd column.

recompression
-------------

With CONFIG_ZRAM_MULTI_COMP, zram can recompress pages using alternative
(secondary) compression algorithms. The basic idea is that alternative
compression algorithm can provide better compression ratio at a price of
(potentially) slower compression/decompression speeds. Alternative compression
algorithm can, for example, be more successful compressing huge pages (those
that default algorithm failed to compress). Another application is idle pages
recompression - pages that are cold and sit in the memory can be recompressed
using more effective algorithm and, hence, reduce zsmalloc memory usage.

With CONFIG_ZRAM_MULTI_COMP, zram supports up to 4 compression algorithms:
one primary and up to 3 secondary ones. Primary zram compressor is explained
in "3) Select compression algorithm", secondary algorithms are configured
using recomp_algorithm device attribute.

Example:::

	#show supported recompression algorithms
	cat /sys/block/zramX/recomp_algorithm
	#1: lzo lzo-rle lz4 lz4hc [zstd]
	#2: lzo lzo-rle lz4 [lz4hc] zstd

Alternative compression algorithms are sorted by priority. In the example
above, zstd is used as the first alternative algorithm, which has priority
of 1, while lz4hc is configured as a compression algorithm with priority 2.
Alternative compression algorithm's priority is provided during algorithms
configuration:::

	#select zstd recompression algorithm, priority 1
	echo "algo=zstd priority=1" > /sys/block/zramX/recomp_algorithm

	#select deflate recompression algorithm, priority 2
	echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm

Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,
which controls recompression.

Examples:::

	#IDLE pages recompression is activated by `idle` mode
	echo "type=idle" > /sys/block/zramX/recompress

	#HUGE pages recompression is activated by `huge` mode
	echo "type=huge" > /sys/block/zram0/recompress

	#HUGE_IDLE pages recompression is activated by `huge_idle` mode
	echo "type=huge_idle" > /sys/block/zramX/recompress

The number of idle pages can be significant, so user-space can pass a size
threshold (in bytes) to the recompress knob: zram will recompress only pages
of equal or greater size:::

	#recompress all pages larger than 3000 bytes
	echo "threshold=3000" > /sys/block/zramX/recompress

	#recompress idle pages larger than 2000 bytes
	echo "type=idle threshold=2000" > /sys/block/zramX/recompress

Recompression of idle pages requires memory tracking.

During re-compression for every page, that matches re-compression criteria,
ZRAM iterates the list of registered alternative compression algorithms in
order of their priorities. ZRAM stops either when re-compression was
successful (re-compressed object is smaller in size than the original one)
and matches re-compression criteria (e.g. size threshold) or when there are
no secondary algorithms left to try. If none of the secondary algorithms can
successfully re-compressed the page such a page is marked as incompressible,
so ZRAM will not attempt to re-compress it in the future.

This re-compression behaviour, when it iterates through the list of
registered compression algorithms, increases our chances of finding the
algorithm that successfully compresses a particular page. Sometimes, however,
it is convenient (and sometimes even necessary) to limit recompression to
only one particular algorithm so that it will not try any other algorithms.
This can be achieved by providing a algo=NAME parameter:::

	#use zstd algorithm only (if registered)
	echo "type=huge algo=zstd" > /sys/block/zramX/recompress

memory tracking
===============

With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the
zram block. It could be useful to catch cold or incompressible
pages of the process with*pagemap.

If you enable the feature, you could see block state via
/sys/kernel/debug/zram/zram0/block_state". The output is as follows::

	  300    75.033841 .wh...
	  301    63.806904 s.....
	  302    63.806919 ..hi..
	  303    62.801919 ....r.
	  304   146.781902 ..hi.n

First column
	zram's block index.
Second column
	access time since the system was booted
Third column
	state of the block:

	s:
		same page
	w:
		written page to backing store
	h:
		huge page
	i:
		idle page
	r:
		recompressed page (secondary compression algorithm)
	n:
		none (including secondary) of algorithms could compress it

First line of above example says 300th block is accessed at 75.033841sec
and the block's state is huge so it is written back to the backing
storage. It's a debugging feature so anyone shouldn't rely on it to work
properly.

Nitin Gupta
ngupta@vflare.org