Especially on systems with little RAM, it can happen from time to time that the available memory becomes scarce and the system starts to use swap memory. As swap memory is typically located on the hard drive, it is relatively slow compared to RAM and thus may impact the system's performance.
A remedy is provided by a technology integrated in the Linux kernel called zRam. It creates a compressing block device directly in the computer's RAM. The kernel first occupies the available RAM and then tries to compress parts of it into zRam. It effectively only moves data within the RAM, which is magnitudes faster than swapping to disk. This technique, therefore, allows more data to be accommodated in memory. Practically, this results in the system not having to swap to a slower hard disk as quickly. The price is a slightly increased processor load which usually has a much smaller impact than swapping to disk.
The installation is quite simple but can make a big impact on the performance of your server if memory is tight.
This tutorial should be adaptable to most Linux Distros since zRam is a kernel function. Yet the activation of the feature/installation of the package might be different. I want to keep the tutorial related to Ubuntu and Debian to keep it short and meaningful. So all you need is a vServer or root server from netcup with Ubuntu or Debian installed and a user with root access.
sudo apt install zram-tools
Edit the configuration file with your favorite text editor, e.g.
sudo editor /etc/default/zramswap:
## EXAMPLE Configuration
## Adjust to your needs
## Specifies amount of zRam devices to create.
## By default, zRamswap-start will use all available cores.
## Specifies the amount of RAM that should be used for zRam
## based on a percentage the total amount of available memory
## Specifies a static amount of RAM that should be used for
## the zRam devices, this is in MiB
## Specifies the priority for the swap devices, see swapon(2)
## for more details.
In this example configuration, we assume that we have a system with 4 cores, all of which are to be used for (de)compression.
We can then tell zRam how much of our RAM should be used. This can either be a percentage of the total amount of available memory (PERCENTAGE) or a fixed value (ALLOCATION). In the example, we decided to use a fixed amount of 4096 MiB of RAM.
PRIORITY sets the priority for the swap device. You normally want to use zRam as the first swap device used, so we keep this at 100 (which is the highest priority).
CORES and ALLOCATION now determine the number of block devices and their respective size (you will see what this means in a moment).
There are various methods to reboot your system. Here is one that works from your terminal:
Now you should be able to see your zRam SWAP by typing
sudo swapon -s, which will show output like this:
Filename Type Size Used Priority
/swapfile file 4194300 0 -2
/dev/zram0 partition 1048572 30472 100
/dev/zram1 partition 1048572 30216 100
/dev/zram2 partition 1048572 30652 100
/dev/zram3 partition 1048572 30772 100
We have a "standard" swap file
/swapfile that is located on our actual hard drive (this is the one we only want to use if we have no other option) and we have four zRam partitions
/dev/zram3 (they equal our configured cores) with the size of ALLOCATION / CORES (4096 MiB / 4). You can also see how much zRam is currently used and the priorities of the swap devices.
Congratulations! You have successfully installed and configured zRam to make better use of your system's resources.
If you want to learn more about how zRam (swap) is used on your server, you should take a look at
/etc/sysctl.conf, where you can configure your swappiness, cache pressure etc.
## SYSCTL ZRAM EXAMPLE Configuration
The configuration above works well on systems with low memory. We start swapping early (as we want to use our zRam as early as possible) and have set a high cache pressure to reduce memory consumption in the long run. For a full explanation of the settings take a look here: https://www.kernel.org/doc/html/latest/admin-guide/sysctl/vm.html
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