Adding a new swap file. How to edit fstab to enable swap after reboot?

Question

I've deleted my existing swap partition due to some partitioning problem. I don't have a swap space now so I've created a swap file with:

dd if=/dev/zero of=/root/myswapfile bs=1M count=1024

Here's after swapon -s:

/root/myswapfile    file    1048572    1320    -1

Now I want to edit my /etc/fstab to enable the swap file after reboot.

Answer 1

These ares the steps to create a swap on a file:

Create a large file e.g. with

sudo mkdir -p /var/cache/swap/   # create a directory that holds the swap file
sudo dd if=/dev/zero of=/var/cache/swap/myswap bs=1M count=4096 # for 4 GByte

Of course any other method of creating a file of defined size would do.

Announce swap to the system

sudo chmod 0600 /var/cache/swap/myswap # only root should have access
sudo mkswap /var/cache/swap/myswap # format as swap
sudo swapon /var/cache/swap/myswap # announce to system

Insert the following line in /etc/fstab for swap from the next boot:

/var/cache/swap/myswap    none    swap    sw    0   0

Note: In case you have your system files on a SSD you may want to consider to hold your swap file on a hard disk location.

Also note: You can use a swap file for hibernation since 2021, but it takes some more work. You need to find the UUID of your root partition & offset of the swapfile and then add resume=UUID=xxx resume_offset=xxx as value of GRUB_CMDLINE_LINUX_DEFAULT in /etc/grub/default, possibly also initramfs (see [https://ubuntuhandbook.org/index.php/2021/08/enable-hibernate-ubuntu-21-10/] for details.)

Additional note for Ubuntu >= 17.04: A swap on file /swapfile is created by default in a new installation (when no swap partition was present). We can manually create a swap partition later if we prefer.

In case we want to replace an existing swap (e.g. partition) with another swap (e.g. on file) we need to remove the old swap with

sudo swapoff -a  # for all

Then remove the swap entry from /etc/fstab or replace it with the new swap respectively.