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zram: Compressed RAM-based block devices
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
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 (
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
Following shows a typical sequence of steps for using zram.
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
-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
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).
#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.
# 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
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.
# 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.
cat /sys/class/zram-control/hot_add
To remove the existing /dev/zramX device (where X is a device id)
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
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
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
============= =============================================================
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
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
============= =============================================================
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
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 partition at this moment.
If admin wants to use incompressible page writeback, they could do 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_MEMORY_TRACKING 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 writeback idle pages from memory to the storage.
If admin want to write a specific page in zram device to 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, he 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 want to limit writeback as per-day 400M, he could do it
like below::
$ 4K_SHIFT=12
$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
$ echo 1 > /sys/block/zram0/writeback_limit_enable
If admins want to allow further write again once the budget is exhausted,
he could do it like below::
$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
If admin wants to see remaining writeback budget since last set::
$ cat /sys/block/zramX/writeback_limit
If admin want to disable writeback limit, he 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, he could
know it via /sys/block/zram0/bd_stat's 3rd column.
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
First column
zram's block index.
Second column
access time since the system was booted
Third column
state of the block:
same page
written page to backing store
huge page
idle page
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
Nitin Gupta