Snapshots are one of the most powerful features of ZFS. A snapshot provides a read-only, point-in-time copy of the dataset. With Copy-On-Write (COW), ZFS creates snapshots fast by preserving older versions of the data on disk… Snapshots preserve disk space by recording just the differences between the current dataset and a previous version… [and] use no extra space when first created, but consume space as the blocks they reference change.