14th April 2016
Recently I had a need to add a swap file to my Debian installation.
However, I am now using btrfs, and – as with any other COW filesystem – it is not possible to simply create a swap file and use it.
There are workarounds (creating a file with a COW attribute removed, and then loop-mounting it), but I just did not like them.
So I have decided to add a swap partition.
It worked amazingly (and very easily), there was even no need to reboot – at all.
I still did restart, just to make sure the system is bootable – and all was perfectly fine.
My initial setup is very simple: a single /dev/sda1 partition on the /dev/sda disk, fully used by btrfs.
Different important paths/mountpoints are btrfs subvolumes, using flat hierarchy.
For this example, let us assume that /dev/sda (and /dev/sda1) is 25GB large, and that I want to add a 2GB swap /dev/sda2 after /dev/sda1.
Brief explanation before we start:
- shrink btrfs filesystem by more than 2GB;
- shrink btrfs partition by 2GB;
- create new 2GB partition for the swap;
- resize btrfs filesystem to full size of its new-size partition;
- initialize swap and turn it on.
Here are the very easy steps! Just make sure you do not make mistakes anywhere
- If your btrfs volume with ID 5 (top level) is a separate mountpoint: mount it now, e.g.
sudo mount /toplevel.
- Take note of your current partition label and UUID:
- Resize btrfs filesystem down (shrink) with a good margin; for example, if I want to add a 2 GB swap, then I can
sudo btrfs fi resize -3g /toplevel– here, I’m shrinking btrfs filesystem by about a gigabyte more than necessary. The process is very quick if you have free space, so you can even use a larger margin – say,
sudo btrfs fi resize -5g /toplevel.
sudo parted, then
resizepart 1(where 1 is the partition number), and answer a few questions:
new_size_here(in our example: 23.0GB),
yes. You can also create a swap partition from parted, then quit parted with
sudo partprobeto let the OS know that partitions have changed.
- I have used cfdisk to create a 2GB swap partition: it has a very simple ncurses UI, and is very intuitive. After creating swap partition, do run
- Resize btrfs filesystem back up to take all of the partition:
sudo btrfs fi resize max /toplevel.
- Simply to be sure, run a scrub:
sudo btrfs scrub start -B -r /toplevel.
- Initialize swap; you can specify uuid and/or label which you may already have in your fstab:
mkswap --label=swap --uuid=your1234-your-uuid-1234-youruuid1234 /dev/sda2.
sudo blkidto make sure your /dev/sda1 UUID stayed the same (or to get swap uuid/label if you haven’t specified any).
- Optionally, add the swap line to your /etc/fstab. Then turn on swap with
That’s it! Amazing, isn’t it? On-the-fly filesystem and partition resizing!