Using a swapfile

ArchWiki article

A swapfile has the same function of a swap partition. The advantage of swapfiles is that they they can be reallocated if need be. Whereas changing size of a partition is extremely difficult (except when using lvm). The performances of a swap file are the same as for a swap partition: Linux kernel ignores the filesystem and accesses the block contents directly on the disk.

• Allocate swapfile: head -c {size (e.g. 4G)} < /dev/urandom > {swapfile (e.g. /swapfile)}. Man page for swapon suggest avoiding using fallocate, as some guides do.
• chmod 600 {swap file}
• mkswap {swapfile}: to format the swap file so that it can be used as swap
• Add the swap file to /etc/fstab: the line should look similar to {swapfile location (e.g. /swapfile)} none swap sw 0 0

Swapfile will be activated on boot. To activate it without rebooting, execute swapon {swapfile}

Removing/resizing a swapfile

To resize a swapfile: delete it and recreate it.

• swapoff {swapfile} to deactivate the swap
• rm {swapfile} to delete the swap
• Delete swapfile line from /etc/fstab (unless you recreate it with the same name)

Setting up hibernation with a swapfile

For the system to be able to hibernate successfully when a swapfile is used, some steps have to e followed.

• filefrag -v {swapfile} Look for the first big number which represent the physical offset of the swap file on the disk. e.g. in 0: 0.. 0: 2193408.. 2193408: 1: it is 2193408.
• Edit /etc/default/grub to add options to GRUB_CMDLINE_LINUX
  • Add resume={partition containing the swap file} For an encrypted system it should be something like /dev/mapper/{root filesystem partition name}
  • Add resume_offset={offset found earlier}
• update-grub
• Reboot the system so that all the parameters are loaded.

It is now possible to hibernate