Creating a Swap File on a Ramdisk with Ubuntu 20.04

August 13, 2021

For some recent work I was looking to negate the latency of storage accesses of swapping in order to more easily measure the kernel’s swap system overhead without worrying about the latency of the underlying block device.

Naively, I initially tried with mount -t ramfs /mnt/ramdisk and simply tried creating a swap file on that mount, but after trying something to the tune of swapon /mnt/ramdisk/swap.img I was met with an Operation not supported. Some google-fu led me to the answer. tmpfs and ramfs don’t support the APIs required to act as a device to read and write a swap image.

However, it is possible (at least if you’re on Ubuntu 20.04 Server) to use the brd kernel module to create a block device at /dev/ram0 which can then be formatted with a filesystem swap file that can be used as a ramdisk. Here is the script:

#!/usr/bin/env bash

# create the mount point directory
sudo mkdir /mnt/ramdisk
# insert kernel module for /dev/ram0 and allocate 2GiB for swapfile
sudo modprobe -C brd.conf brd
# Format our ram device
sudo dd if=/dev/zero of=/dev/ram0 bs=1M count=2048
# create ext4 filesystem and mount (2GiB)
sudo mkfs.ext4 -b 4096 /dev/ram0 536870912
# mount the block device
sudo mount /dev/ram0 /mnt/ramdisk
# create swapfile
sudo dd if=/dev/zero of=/mnt/ramdisk/swap.img bs=1M count=2048
# setup permissions and enable the swap file
sudo chown root:root /mnt/ramdisk/swap.img
sudo chmod 600 /mnt/ramdisk/swap.img
sudo mkswap /mnt/ramdisk/swap.img
sudo swapon /mnt/ramdisk/swap.img
swapon -s

And the file for brd.conf for the modprobe command contains parameters to ensure the ramdisk is at least 2GiB (the desired size for my swap file):

options brd rd_size=2147483648
options brd rd_nr=1

Frankly, the latency using a ramdisk directly here was pretty disappointing compared to an NVMe SSD, but I will potentially explore that issue in a later post.

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