Snapshot (computer storage) - Wikipedia
https://en.wikipedia.org/wiki/Snapshot_(computer_storage)Some Unix systems have snapshot-capable logical volume managers. These implement copy-on-write on entire block devices by copying changed blocks—just before they are to be overwritten within "parent" volumes—to other storage, thus preserving a self-consistent past image of the block device. Filesystems on such snapshot images can later be mounted as if they were on a read-only media.