The Book of Xen

Copy-on-Write and Snapshots

One of the other niceties that a real storage option gives you is copy-on-write, which means that, rather than the domU overwriting a file when it's changed, the backend instead transparently writes a copy elsewhere.^[29] As a corollary, the original filesystem remains as a snapshot, with all modifications directed to the copy-on-write clone.

This snapshot provides the ability to save a filesystem's state, taking a snapshot of it at a given time or at set intervals. There are two useful things about snapshots: for one, they allow for easy recovery from user error.^[30] For another, they give you a checkpoint that's known to be consistent—it's something that you can conveniently back up and move elsewhere. This eliminates ...