AIX Tip of the Week

Subject: Online Backups

Audience: Administration

Date: July 6, 2004

There are two ways to do on-line filesystem backup in AIX, depending on whether it is a JFS or JFS2 filesystem.

For JFS filesystems (AIX 5.1, 5.2) use the "chfs" command with the "splitcopy" option**. This splits off one of the mirrored filesystem copies, and mounts it on a separate, static filesystem. The static filesystem is used for the backup, while the primary copy runs production. After backup completes, the split copy is manually reintegrated with production in the background ("rmfs static copy").

For JFS2 (AIX 5.2), use the "backsnap" command (smit backsnap) which does the entire snapshot, backup and reintegration in one command. "backsnap" uses less disk space as it doesn't require a full mirror copy. Instead it uses a small snapshot filesystem (2-15% of the filesystem to be backed up). The snapshot is a "point in time" image of the filesystem, and is the basis for backup. The backup can be either full (by name) or incremental (by inode). In either case, the backup runs in the background. When completed, there is an option to remove the snapshot. For more information, see:

Systems Management Concepts (See section: "Understanding JFS Online Backups and JFS2 Snapshots")

(**Comment: the JFS log has to be mirrored for "chfs -a splitcopy" to work)

Bruce Spencer,

July 6, 2004