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Comment by optionalsquid | Hacker News Reader

It also doesn't sound like a particularly good idea on BTRFS, judging by the linked documentation:

https://btrfs.readthedocs.io/en/latest/Swapfile.html

And since you should probably be running a modern CoW filesystem then you probably shouldn't be using swapfiles ... is the conclusion I arrived at anyway. Either install multiple times more RAM than you need and enable zswap or else set aside a swap partition as has always been done.

Truth be told—- I don’t use swap and set `vm.swappiness=0` but in these difficult times, swap may be the hero we need.

Swapfiles on BTRFS seem to work great if you allocate them early (with the command linked in your article) and put them in a different subvolume than the rest of your system.

BTRFS in general seems more I/O constrained than ext4 or xfs so it's probably still worse to put a swapfile on a BTRFS partition, but so far my systems are doing fine.

> BTRFS in general seems more I/O constrained than ext4 or xfs so it's probably still worse to put a swapfile on a BTRFS partition,

AFAIK, on Linux swap bypasses the filesystem. It asks the filesystem for the extents which are used by the swapfile, and does the I/O directly on the block device. This explains the many restrictions swapfiles have on btrfs (the file can't be copy-on-write, can't be mirrored, etc), but it also means the speed for a swapfile will be the same on btrfs as on ext4 or xfs (assuming a similar layout, that is, the file is not too fragmented; IIRC, the kernel prints the number of extents when you do a swapon, so you can see when it's too fragmented).

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