Autarchy of the Private Cave

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:

1. shrink btrfs filesystem by more than 2GB;
2. shrink btrfs partition by 2GB;
3. create new 2GB partition for the swap;
4. resize btrfs filesystem to full size of its new-size partition;
5. initialize swap and turn it on.

Here are the very easy steps! Just make sure you do not make mistakes anywhere
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In short: sudo mkswap -L new_swap_label -U old_swap_UUID /dev/sd_swap_device.
If you don’t care about the UUID: just sudo mkswap -L new_swap_label /dev/sd_swap_device.

Step-by-step:
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