Sunday, 5 February 2023

How to enable zRAM in Rocky Linux 9

How to enable zRAM in Rocky Linux 9

In this Linux tutorial, you will learn how to enable zRAM in Rocky Linux 9 or other Red Hat based Linux distros.


What is zRAM?:

zRAM, formerly called compcache, is a Linux kernel module for creating a compressed block device in RAM, i.e. a RAM disk with on-the-fly disk compression. The block device created with zRAM can then be used for swap (named as zswap) or as general-purpose RAM disk.

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>/


Environment Specification:

We are using a minimal Rocky Linux 9 virtual machine with following specifications.

  • CPU - 3.4 Ghz (2 cores)
  • Memory - 2 GB
  • Storage - 20 GB
  • Operating System - Rocky Linux release 9.1 (Blue Onyx)
  • Hostname – rocky9.centlinux.com
  • IP Address -

Enable zRAM Linux Kernel Module:

By using a ssh client, login to your Linux server as root user.

Create a file with following content, to enable zRAM module during loading of your Linux Kernel.

# cat > /etc/modules-load.d/zram.conf << EOF
> zram

Create configuration file for zRAM Kernel Module.

In this configuration file, you can set the number of zRAM block devices (num_devices=x) for your Linux server.

# cat > /etc/modprobe.d/zram.conf << EOF
> options zram num_devices=1

Create a udev rule and specify the size of your zRAM device therein.

# cat > /etc/udev/rules.d/99-zram.rules << EOF
> KERNEL=="zram0", ATTR{disksize}="2G",TAG+="systemd"

Open /etc/fstab file by using vim text editor.

# vi /etc/fstab

Find and comment the following line to disable automount of default swap partition. If you have more than one swap partitions then disable all of them.

#/dev/mapper/rl-swap     none                    swap    defaults        0 0

Create a systemd unit for zRAM to enable zswap partition on your Rocky Linux server.

By using vim text editor create a systemd unit.

# vi /etc/systemd/system/zram.service

Add following directives in this file.

Description=Swap with zram

ExecStartPre=/sbin/mkswap /dev/zram0
ExecStart=/sbin/swapon /dev/zram0
ExecStop=/sbin/swapoff /dev/zram0


Enable zram.service by executing following command.

# systemctl enable zram.service
Created symlink /etc/systemd/system/multi-user.target.wants/zram.service → /etc/systemd/system/zram.service.

Reboot your Linux server to let it start with the new zswap partition.

# reboot

After reboot, check the status of your zram block devices by executing zramctl command.

# zramctl
/dev/zram0 lzo-rle         2G   4K   74B   12K       1 [SWAP]

Check the status of zswap memory by using free command at Linux bash prompt.

# free -m
               total        used        free      shared  buff/cache   available
Mem:            1743         392        1323           5         174        1351
Swap:           2047           0        2047

Currently, the zswap memory is not in used, because we are using a test machine with almost zero load on it. The output may vary on a producion grade server.

You need to write a C language program, to create some imaginery load on your Rocky Linux server.

You can use vim text editor to write your C language program.

# vi ~/memeater.c

Add following lines of code in this file.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char** argv) {
    int max = -1;
    int mb = 0;
    int multiplier = 100; // allocate 1 MB every time unit. Increase this to e.g.100 to allocate 100 MB every time unit.
    char* buffer;

    if(argc > 1)
        max = atoi(argv[1]);

    while((buffer=malloc(multiplier * 1024*1024)) != NULL && mb != max) {
        memset(buffer, 1, multiplier * 1024*1024);
        printf("Allocated %d MB\n", multiplier * mb);
        sleep(1); // time unit: 1 second
    return 0;

You may need to install C language compiler to compile your program.

Use dnf command to install gcc package.

# dnf install -y gcc

Now, use gcc command to compile your C language program.

# gcc ~/memeater.c -o ~/memeater

Adjust the swappiness of your Linux server to 100% for immediate result.

# sysctl vm.swappiness=100

Now execute memeater command.

# ~/memeater

Open another ssh session and check the usage of zswap partition.

# free -m
               total        used        free      shared  buff/cache   available
Mem:            1743        1713          30           13        473          30
Swap:           2047         834        1213

Our zswap partition is in use now.

Reboot your Linux server now.

# reboot



In this Linux tutorial, you have learned how to enable zRAM on Rocky Linux 9 or other Red Hat based Linux distros.

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