BTRFS – How to Make Free Space Available After Resize

Thanks to those who posted, but your answers were way too vague to be of any help.

After hours of Googling, I finally found a guide(link redacted) providing step-by-step instructions on how to shrink my filesystem, logical volumes, and physical volumes. The trick that most guides miss is the need to actually delete the physical partitions, and recreate them with the correct size, because parted is unable to resize lvm partitions.

I then found this documentation on qemu-img, which explains how to shrink a raw-formatted virtual disk image by running:

sudo qemu-img resize /var/lib/libvirt/images/myvm.img 255G

Most likely, you're running into a problem where BTRFS has to allocate a bit of metadata before it can remove the file. One of BTRFS's weak spots is its handling of the out-of-space condition; improving behavior in this area is one of the project's priorities.

One suggestion on the btrfs wiki is to clobber the file instead of removing it.

#instead of this
rm -f ./some_file

# do this
true >| ./some_file

That'll remove the file's contents without changing the directory entry. Once you've worked your way out of the corner you're in, you can then delete files like normal. If that gives you trouble, you may want to temporarily remount with the nodatacow option, which turns off the copy-on-write behavior. But... not sure for certain if that will help or not.

In general, though: don't run a BTRFS filesystem dry. It's still pre-production software, and the corner cases are a little rough.