What-What?? What Is Going On With This Snapshot?!?

• We can continue working with the original device, without changing the data in the snapshot.
• COW (Copy-On-Write) - Every time data is written to the original device, the Linux kernel copies the old data to the snapshot's device, and only then updates the original device.
• Thus, the first write to a snapshot is translated into a read (from the origin), a write (to the snapshot) and another write (to the origin).
• The kernel keeps an on-disk hash-table of the chunks that were copied to the snapshot.
• This is used both for reads, and to avoid redundant copies.

Originally written by guy keren