The [linux] volume manager also allows reducing the amount of disk space allocated to a logical volume, but there are a couple requirements. First, the volume must be unmounted. Second, the filesystem itself must be reduced in size before the volume on which it resides can be reduced.
https://opensource.com/business/16/9/linux-users-guide-lvm
Each volume within a volume group is segmented into small, fixed-size chunks called extents. The size of the extents is determined by the volume group (all volumes within the group conform to the same extent size).
The extents on a physical volume are called physical extents, while the extents of a logical volume are called logical extents. A logical volume is simply a mapping that LVM maintains between logical and physical extents.
The logical extents that are presented as a unified device by LVM do not have to map to continuous physical extents.
https://www.digitalocean.com/community/tutorials/an-introduction-to-lvm-concepts-terminology-and-operations
Daniel Velasco (notes?) 