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ZRam: Compress Your RAM

ZRam is a Linux kernel module that creates a compressed block device in RAM. Think of it as a way to "squeeze" more data into your physical memory. Instead of swapping out to a slow disk when RAM gets full, Linux can swap into this compressed ZRam device, which is significantly faster.

This is especially great for systems with limited RAM, as it trades a little bit of CPU for a lot more usable memory space.

Installation

Debian makes this incredibly easy with the zram-tools package. It handles all the setup scripts for us.

We will install the package:

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sudo apt-get install zram-tools

Configuration

By default, the package sets up a basic configuration, but let's tweak it to get the best performance. We want to use the zstd compression algorithm (which offers a great balance of speed and compression ratio) and dedicate a healthy chunk of RAM to it.

We will edit the configuration file /etc/default/zramswap:

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# Compression algorithm selection
# speed: lz4 > zstd > lzo
# compression: zstd > lzo > lz4
# This is not inclusive of all available algorithms.
# See /sys/block/zram0/comp_algorithm (when zram module is loaded) to see
# what your kernel provides.
ALGO=zstd

# Specifies the amount of RAM that should be used for zram
# based on a percentage the total amount of available memory
PERCENT=60

# Specifies a static amount of RAM that should be used for zram
# This is in MiB
#SIZE=256

# Specifies the priority for the swap devices
# See swapon(8) for more details
#PRIORITY=100

Once we've saved our changes, we need to restart the service to apply them:

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sudo systemctl restart zramswap

Verification

Let's check if our ZRam device is up and running. The zramctl tool gives us a nice summary. You should see output similar to this, showing the algorithm (zstd) and the disk size:

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$ zramctl
NAME       ALGORITHM DISKSIZE  DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lz4           1.2G 28.8M   18M 18.6M       2 [SWAP]

We can also verify that it's being heavily utilized as a high-priority swap device:

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$ swapon --show
NAME       TYPE      SIZE  USED PRIO
/dev/zram0 partition 1.2G 29.3M  100

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