I need to add a swap file to a running VM, but I do now want to attach a new disk.
I’ll use a file on my VM to use as swapfile.

We are going to use a file as swap, mounting it as a block device (this will allow us, eventually, to use dm-crypt and encrypt the swap partition)

Create a swap file

Create 1Gb file filled with random stuff

dd if=/dev/urandom of=/swapfile.swap bs=1M count=1024
chmod 600 /root/swapfile.swap
mkswap /swapfile.swap

Create systemd service to load swap

vim /etc/systemd/system/fileswap.service

Fill the file with this lines

[Unit]
Description=Mount my swap

[Service]
Type=oneshot
Environment="SWAPFILE=/swapfile.swap"
RemainAfterExit=true

ExecStartPre=/usr/sbin/losetup /dev/loop0 ${SWAPFILE}
ExecStart=/sbin/swapon /dev/loop0

ExecStop=/sbin/swapoff /dev/loop0
ExecStopPost=/usr/sbin/losetup -d /dev/loop0

[Install]
WantedBy=multi-user.target

Enable and test

systemctl daemon-reload

#enable service
systemctl enable fileswap.service

# start service
systemct start fileswap.service

# stop service
systemctl stop fileswap.service

Tips

If your file system is subject to COW it is a good idea to disable cow operations on swap file

lsattr /swapfile.swap

chattr +C /swapfile.swap

lsattr /swapfile.swap

chattr (C option)

A file with the ‘C’ attribute set will not be subject to copy-on-write updates. This flag is only supported on file systems which perform copy-on-write. (Note: For btrfs, the ‘C’ flag should be set on new or empty files. If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable. If the ‘C’ flag is set on a directory, it will have no effect on the directory, but new files created in that directory will have the No_COW attribute set.)

Plain and easy alternative

We can use a file directly and use it as swapfile

dd if=/dev/urandom of=/swapfile.swap bs=1M count=1024
chmod 600 /swapfile.swap
mkswap /swapfile.swap
swapon /swapfile.swap

Add this line in /etc/fstab

/swapfile.swap    none    swap    sw    0   0