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):

  1. using zram and zram_control sysfs attributes

  2. 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 shows only compression algorithms that are supported by zram.

It is also possible to pass algorithm specific configuration parameters:

#set compression level to 8
echo "zstd level=8" > /sys/block/zram0/comp_algorithm

Note that comp_algorithm also supports algo=name format:

#set compression level to 8
echo "algo=zstd level=8" > /sys/block/zram0/comp_algorithm

Certain compression algorithms support pre-trained dictionaries, which significantly change algorithms’ characteristics. In order to configure compression algorithm to use external pre-trained dictionary, pass full path to the dictionary along with other parameters:

#pass path to pre-trained dictionary
echo "algo=zstd dict=/etc/dictioary" > /sys/block/zram0/comp_algorithm

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 Block layer statistics in /sys/block/<dev>/stat 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

  1. the number of pages freed because of swap slot free notifications

  2. 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

The recomp_algorithm also supports algorithm configuration parameters, e.g. compression level and pre-trained dircionary:

#pass compression level
echo "algo=zstd level=8" > /sys/block/zramX/recomp_algorithm

#pass path to pre-trained dictionary
echo "algo=zstd dict=/etc/dictioary" > /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

It is also possible to limit the number of pages zram re-compression will attempt to recompress::

echo "type=huge_idle max_pages=42" > /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