[cmurf]

A copy on write file system has this potential problem because nothing is overwritten. To delete anything requires space to write the metadata change reflecting the deletion, and before the data extends can be freed the change must be committed to stable media.

It's been years since Btrfs introduced "global reserve" which reserves enough metadata space to ensure it's possible to delete files on full file systems. But an old work around for this is to add a small device to the Btrfs volume, making is a 2 device volume. It could be a USB stick, a zram device (ramdisk), partition, or even a loop mounted file on some other file system. Delete the files, and now you can remove the temporary 2nd device.