Where to get btrfs-convert on 18.04?

btrfs-convert was a part of "btrfs-tools" package, which in Bionic will be provided as "btrfs-progs".

However if you check the list of "btrfs-progs" files, you would find out that the man page of btrfs-convert has been included but not the binary itself.

There is also a bug report: btrfs-convert executable is not included in btrfs-progs

Also it might be dropped because of: btrfs-progs: please stop shipping btrfs-convert

You have to compile it from source I guess.

I grabbed a copy from suse and used that ( https://opensuse.pkgs.org/15.0/opensuse-oss/btrfsprogs-4.15-lp150.1.2.x86_64.rpm.html ). (All my data's backed up so I'm not so concerned about the unreliability.

EDIT: as I was warned, I basically got a filesystem that didn't really work (Would go read-only just after booting)

Ran into this bug: https://bugzilla.kernel.org/show_bug.cgi?id=194667

I second the above advice, currently btrfs-convert doesn't work

Instead of btrfs-convert, you could try to use fstransform.

Like the other posters, btrfs-convert produced a broken filesystem for me while fstransform did the job. It says the support for btrfs is still untested, but you can run it with an according parameter to do it anyway.

Do not forget to back up data! As always.