Thinpool_migrator is a special purpose utility designed to migrate an ext4 stateful partition to an LVM layout where the entire partition resides inside a thinpool. The thinpool migrator achieves this by resizing the filesystem down by ~2% of disk size and generating thinpool metadata to make the current filesystem appear as a thinly provisioned logical volume.
Thinpool migrator is inspired by g2p/lvmify (https://github.com/g2p/lvmify) that is now a part of g2p/blocks (https://github.com/g2p/blocks) with a slightly more convoluted step of putting a thinpool underneath the logical volume.