COW snapshot
also: copy-on-write snapshot, COW, snapshot
A Copy-on-Write snapshot is a space-efficient point-in-time copy of a filesystem or volume that only stores changes made after the snapshot is created, not a full duplicate of the original data.
Copy-on-Write (COW) snapshots leverage a technique where the original data blocks are shared between the snapshot and the source filesystem. When data is modified, the filesystem writes the changes to new blocks rather than overwriting the originals, keeping the snapshot's view of the data unchanged.
This approach is dramatically more efficient than traditional full backups. Instead of consuming double the disk space for a snapshot, COW snapshots typically use only the space needed to store modifications, making them ideal for frequent backup strategies and rapid point-in-time recovery.
Common on modern filesystems like Btrfs, LVM, and ZFS, COW snapshots are frequently used for backup, testing, and recovery scenarios. For example, a sysadmin might create a COW snapshot of /home before a risky system update, allowing rollback if problems occur.