> 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 bypass

> 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).