ZFS compresses recordsize or volblocksize chunks down to some whole number of disk blocks, as determined by 1 >> ashift. In practice, this typically means that each 128k chunk gets compressed to some number of sequential 512 or 4096 byte blocks. These compressed blocks are referenced by block pointers that contain flags indicating compression and what type.

afaik, ZFS will read an entire Record at a time- and that's the same granularity as its compression.

It uses some form of keyframing. Not entirely free.