Take advantage of compressed RAM based block devices to create temporary filesystems and swap disks.
Preliminary information
Operating system.
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.1 LTS
Release: 20.04
Codename: focal
Verify that zram module is available.
$ modinfo zram
modinfo: ERROR: Module zram not found.
Install extra modules for the running kernel.
$ sudo apt install linux-modules-extra-$(uname -r)
Inspect module information.
$ modinfo zram
filename: /lib/modules/5.4.0-54-generic/kernel/drivers/block/zram/zram.ko
description: Compressed RAM Block Device
author: Nitin Gupta <ngupta@vflare.org>
license: Dual BSD/GPL
srcversion: 74320660DB5936B392634A5
depends:
retpoline: Y
intree: Y
name: zram
vermagic: 5.4.0-54-generic SMP mod_unload
sig_id: PKCS#7
signer: Build time autogenerated kernel key
sig_key: 47:AF:FC:F1:86:66:29:ED:36:CF:32:E5:45:0F:C1:C8:85:38:3E:AF
sig_hashalgo: sha512
signature: 30:27:58:D1:C9:B7:7B:E2:2D:92:3B:30:D1:AC:DC:A5:82:5D:20:8D:
A0:59:BE:82:BB:97:47:5F:F8:0F:66:41:70:2B:D4:CE:75:DB:62:6A:
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BA:04:05:80:C5:68:8C:79:60:4B:E3:2D:66:28:DB:7B:25:2C:1F:7C:
F9:B9:56:1D:36:AE:F0:30:DD:F2:F2:1E
parm: num_devices:Number of pre-created zram devices (uint)Load zram module. You can use num_devices parameter to create multiple devices immediately, but this is purely optional as you can always get the additional free device, one at a time.
$ sudo modprobe zram
Ensure that zram module will load at boot.
$ echo zram | sudo tee /etc/modules-load.d/zram.conf
If required, set num_devices parameter at boot.
$ echo options zram num_devices=4 | sudo tee /etc/modprobe.d/zram.conf
Familiarize yourself with the zramctl utility, which is included in the util-linux package.
$ zramctl --help
Usage:
zramctl [options] <device>
zramctl -r <device> [...]
zramctl [options] -f | <device> -s <size>
Set up and control zram devices.
Options:
-a, --algorithm lzo|lz4|lz4hc|deflate|842 compression algorithm to use
-b, --bytes print sizes in bytes rather than in human readable format
-f, --find find a free device
-n, --noheadings don't print headings
-o, --output <list> columns to use for status output
--output-all output all columns
--raw use raw status output format
-r, --reset reset all specified devices
-s, --size <size> device size
-t, --streams <number> number of compression streams
-h, --help display this help
-V, --version display version
Available output columns:
NAME zram device name
DISKSIZE limit on the uncompressed amount of data
DATA uncompressed size of stored data
COMPR compressed size of stored data
ALGORITHM the selected compression algorithm
STREAMS number of concurrent compress operations
ZERO-PAGES empty pages with no allocated memory
TOTAL all memory including allocator fragmentation and metadata overhead
MEM-LIMIT memory limit used to store compressed data
MEM-USED memory zram have been consumed to store compressed data
MIGRATED number of objects migrated by compaction
MOUNTPOINT where the device is mounted
For more details see zramctl(8).Create a temporary filesystem
Get a first free device. Use sudo, always use sudo to find and initialize the device or you will end up with a missing entry in /dev which can be hard to debug.
$ sudo zramctl --find
/dev/zram0
Create a new device with 1G disk size, using lz4 compression algorithm and two compression streams, which should be equal to the number of CPUs.
$ sudo zramctl --size 1G --algorithm lz4 --streams 2 /dev/zram0
Create a filesystem on this device with usage type indicating the need of inodes. The funny part is that it is very easy to hit limits on such small filesystems. This will allow 262144 instead of 65536 inodes.
$ sudo mkfs.ext2 -T news /dev/zram0
mke2fs 1.45.5 (07-Jan-2020)
Discarding device blocks: done
Creating filesystem with 262144 4k blocks and 65536 inodes
mke2fs 1.45.5 (07-Jan-2020)
Discarding device blocks: done
Creating filesystem with 262144 4k blocks and 262144 inodes
Filesystem UUID: 89ef9d1f-8c6b-46dd-891f-ae5b6e2795c4
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
Mount created filesystem and use it.
$ sudo mkdir /media/temporary
$ sudo mount /dev/zram0 /media/temporary/
Display usage information.
$ zramctl --output-all
NAME DISKSIZE DATA COMPR ALGORITHM STREAMS ZERO-PAGES TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT /dev/zram0 1G 476.9M 154.7M lz4 2 8 162.4M 0B 162.4M 0B /media/temporary
This means that we are using 162.4M of memory to keep 476.9M data.
To remove device unmount it and reset.
$ sudo umount /dev/zram0
$ sudo zramctl --reset /dev/zram0
Create a temporary swap disk
Get a first free device.
$ sudo zramctl --find
/dev/zram1
Create a new device with 512M disk size, using lz4 compression algorithm and two compression streams.
$ sudo zramctl --size 512M --algorithm lz4 --streams 2 /dev/zram1
Create swap area.
$ sudo mkswap /dev/zram1
Setting up swapspace version 1, size = 512 MiB (536866816 bytes)
no label, UUID=f72a24f4-7b4f-45c8-bff5-38401af6a5ee
Activate swap using a high priority.
$ sudo swapon --priority 100 /dev/zram1
Display swap summary.
$ swapon --summary
Filename Type Size Used Priority /dev/zram1 partition 524284 256 100
Display usage information.
$ zramctl --output-all
NAME DISKSIZE DATA COMPR ALGORITHM STREAMS ZERO-PAGES TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT /dev/zram1 512M 68K 11.2K lz4 2 0 136K 0B 136K 0B [SWAP] /dev/zram0 1G 667.5M 245.9M lz4 2 16 257.4M 300M 257.4M 0B /media/temporary
To remove device disable it and reset.
$ sudo swapoff /dev/zram1
$ sudo zramctl --reset /dev/zram1
Additional notes
Read zram: Compressed RAM based block devices documentation for more information.
Every option used to create zram device is available in the sys filesystem.
$ ls /sys/block/zram0/
alignment_offset comp_algorithm disksize hidden integrity mm_stat reset subsystem writeback_limit_enable backing_dev compact events holders io_stat power ro trace bd_stat debug_stat events_async idle max_comp_streams queue size uevent bdi dev events_poll_msecs inflight mem_limit range slaves writeback capability discard_alignment ext_range initstate mem_used_max removable stat writeback_limit
You can use it to impose a memory limit which can be handy for a filesystem, but do not use it for swap disk.
$ echo 300M | sudo tee /sys/block/zram0/mem_limit
Create device.
$ sudo cat /sys/class/zram-control/hot_add
4
Remove specific device.
$ echo 3 | sudo tee /sys/class/zram-control/hot_remove
3
We created zram devices up to zram4, and removed zram3.
$ find /dev -maxdepth 1 -name "zram*"
/dev/zram4
/dev/zram2
/dev/zram1
/dev/zram0