Introduction
The ZRAM module in the Linux kernel creates a memory-backed block device that stores its content in a compressed format. It offers users the choice of compression algorithms such as lz4, zstd, or lzo. These algorithms differ in compression ratio and speed, with zstd providing the best compression but being slower, while lz4 offers higher speed but lower compression.
Using ZRAM as Swap
One interesting use case for ZRAM is utilizing it as swap space in the system. There are two utilities available for configuring ZRAM as swap: zram-tools and systemd-zram-generator. However, Debian Bullseye lacks systemd-zram-generator, making zram-tools the only option for Bullseye users. While it's possible to use systemd-zram-generator by self-compiling or via cargo, I preferred using tools available in the distribution repository due to my restricted environment.
Installation
The installation process is straightforward. Simply execute the following command:
apt-get install zram-tools
Configuration
The configuration involves modifying a simple shell script file /etc/default/zramswap sourced by the /usr/bin/zramswap script. Here's an example of the configuration I used:
# Compression algorithm selection
# Speed: lz4 > zstd > lzo
# Compression: zstd > lzo > lz4
# This is not inclusive of all the algorithms available in the latest kernels
# See /sys/block/zram0/comp_algorithm (when the zram module is loaded) to check
# the currently set and available algorithms for your kernel [1]
# [1] https://github.com/torvalds/linux/blob/master/Documentation/blockdev/zram.txt#L86
ALGO=zstd
# Specifies the amount of RAM that should be used for zram
# based on a percentage of the total available memory
# This takes precedence and overrides SIZE below
PERCENT=30
# Specifies a static amount of RAM that should be used for
# the ZRAM devices, measured in MiB
# SIZE=256000
# Specifies the priority for the swap devices, see swapon(2)
# for more details. A higher number indicates higher priority
# This should probably be higher than hdd/ssd swaps.
# PRIORITY=100
I chose zstd as the compression algorithm for its superior compression capabilities. Additionally, I reserved 30% of memory as the size of the zram device. After modifying the configuration, restart the zramswap.service to activate the swap:
systemctl restart zramswap.service
Using systemd-zram-generator
For Debian Bookworm users, an alternative option is systemd-zram-generator. Although zram-tools is still available in Debian Bookworm, systemd-zram-generator offers a more integrated solution within the systemd ecosystem. Below is an example of the translated configuration for systemd-zram-generator, located at /etc/systemd/zram-generator.conf:
# This config file enables a /dev/zram0 swap device with the following
# properties:
# * size: 50% of available RAM or 4GiB, whichever is less
# * compression-algorithm: kernel default
#
# This device's properties can be modified by adding options under the
# [zram0] section below. For example, to set a fixed size of 2GiB, set
# `zram-size = 2GiB`.
[zram0]
zram-size = ceil(ram * 30/100)
compression-algorithm = zstd
swap-priority = 100
fs-type = swap
After making the necessary changes, reload systemd and start the systemd-zram-setup@zram0.service:
systemctl daemon-reload
systemctl start systemd-zram-setup@zram0.service
The systemd-zram-generator creates the zram device by loading the kernel module and then creates a systemd.swap unit to mount the zram device as swap. In this case, the swap file is called zram0.swap.
Checking Compression and Details
To verify the effectiveness of the swap configuration, you can use the zramctl command, which is part of the util-linux package. Alternatively, the zramswap utility provided by zram-tools can be used to obtain the same output.
During my testing with synthetic memory load created using stress-ng vm class I found that I can reach upto 40% compression ratio.
Memory Overcommit
Another use case I was looking for is allowing the launching of applications that require more memory than what is available in the system. By default, the Linux kernel attempts to estimate the amount of free memory left on the system when user space requests more memory (vm.overcommit_memory=0). However, you can change this behavior by modifying the sysctl value for vm.overcommit_memory to 1.
To demonstrate this, I ran a test using stress-ng to request more memory than the system had available. As expected, the Linux kernel refused to allocate memory, and the stress-ng process could not proceed.
free -tg ──(Mon,Jun19)─┘
total used free shared buff/cache available
Mem: 31 12 11 3 11 18
Swap: 10 2 8
Total: 41 14 19
sudo stress-ng --vm=1 --vm-bytes=50G -t 120 ──(Mon,Jun19)─┘
stress-ng: info: [1496310] setting to a 120 second (2 mins, 0.00 secs) run per stressor
stress-ng: info: [1496310] dispatching hogs: 1 vm
stress-ng: info: [1496312] vm: gave up trying to mmap, no available memory, skipping stressor
stress-ng: warn: [1496310] vm: [1496311] aborted early, out of system resources
stress-ng: info: [1496310] vm:
stress-ng: warn: [1496310] 14 System Management Interrupts
stress-ng: info: [1496310] passed: 0
stress-ng: info: [1496310] failed: 0
stress-ng: info: [1496310] skipped: 1: vm (1)
stress-ng: info: [1496310] successful run completed in 10.04s
By setting vm.overcommit_memory=1, Linux will allocate memory in a more relaxed manner, assuming an infinite amount of memory is available.
Conclusion
ZRAM provides disks that allow for very fast I/O, and compression allows for a significant amount of memory savings. ZRAM is not restricted to just swap usage; it can be used as a normal block device with different file systems.
Using ZRAM as swap is beneficial because, unlike disk-based swap, it is faster, and compression ensures that we use a smaller amount of RAM itself as swap space.
Additionally, adjusting the memory overcommit settings can be beneficial for scenarios that require launching memory-intensive applications.
Note: When running stress tests or allocating excessive memory, be cautious about the actual memory capacity of your system to prevent out-of-memory (OOM) situations.
Feel free to explore the capabilities of ZRAM and optimize your system's memory management. Happy computing!