UNIX Filesystems: Evolution, Design, and Implementation

Frozen-Image Technology

All of the traditional tools described so far can operate on a filesystem that is mounted and in use. Unfortunately, this can lead to backing up some files that are in the process of being written. If files are being changed while the backup runs, an inconsistent image will likely be written to tape or other media.

Ideally, a backup should be run when there is no activity to the filesystem, allowing all files backed up to be in a consistent state. The system administrator does not, however, want to unmount a busy filesystem just to perform a backup. This is where stable snapshot mechanisms come into play.

A stable snapshot, or frozen image, is a consistent copy of a filesystem that allows a backup application to back up files that are not changing. Even though there still may be activity to the filesystem, the frozen image is guaranteed to be a consistent replica of the filesystem at the time the frozen image was taken.

The following sections describe the two different types of frozen images: snapshots that are not persistent across reboots and snapshots that are persistent across reboots.

Note that there are a number of terms that describe the same concept. Snapshots, frozen-images, and point-in-time copies are used interchangeably in the storage industry to refer to the same thing, a stable image of the filesystem.

Nonpersistent Snapshots

The goal behind any snapshotting technology is to provide a frozen image of the filesystem for the purpose of performing ...