Linux Server Hacks, Volume Two

Hack #48. Create a Copy-on-Write Snapshot of an LVM Volume

Logical volumes don't just provide a great way to supply flexible storage— they can also provide a great way to preserve files that have changed recently, simplifying restores and reducing restore requests.

A snapshot is a copy of a logical volume that reflects the contents of that logical volume when the snapshot was created. With a copy-on-write snapshot, each time a file changes in the original volume, the contents of the original file (as of the time that the snapshot was made) are preserved in the snapshot volume. In other words, the complete contents of the original file are copied to the snapshot volume when you write changes to the file in the original volume. Implementing a copy-on-write volume to track changed files is like having a built-in backup mechanism, because it provides you with a point-in-time copy of the filesystem that is contained on your logical volume. This copy of your filesystem can then be used for retrieving files that have accidentally been deleted or modified. For system administrators, copy-on-write snapshots can be particularly useful in preserving the original copies of system configuration files (just in case you ever make a mistake). However, their real beauty is in preserving copies of volumes containing users' home directories. I've found that taking a nightly snapshot of the logical volume that contains the users' home directories and automatically mounting it enables most users to ...