UNIX Filesystems: Evolution, Design, and Implementation

Filesystem Backup

Backing up a filesystem to tape or other media is one area that is not typically well documented in the UNIX world. Most UNIX users are familiar with commands such as tar and cpio, which can be used to create a single archive from a hierarchy of files and directories. While this is sufficient for creating a copy of a set of files, such tools operate on a moving target—they copy files while the files themselves may be changing. To solve this problem and allow backup applications to create a consistent image of the filesystem, various snapshotting techniques have been employed.

This chapter describes the basic tools available at the UNIX user level followed by a description of filesystem features that allow creation of snapshots (also called frozen images). The chapter also describes the techniques used by hierarchical storage managers to archive file data based on various policies.