Backup, Progressive Incremental Backups
For IT Pros: This page explains Progressive Incremental Backups, the backup method used by IBM Tivoli Storage Manager. The Enterprise Backup service is powered by the IBM Tivoli Storage Manager product.
Progressive Incremental Backups Explained
Tivoli Storage Manager uses a method for backups they call Progressive Incremental Backups. Essentially, a full backup is only taken once - the first time the system is introduced to backups. Subsequent backups are incremental backups, that only backup files that have changed or are new.
TSM tracks this data internally by considering every file that exists on the system during the most recent backup as active. These files are always present in TSM and are never aged off (thus making TSM a mirror of the live system at the last backup point.) If a file is changed on the system for the next backup, TSM makes that new version active and marks the previously stored version as inactive. The policies that define data retention in TSM define how many inactive copies are stored in TSM and for how long. Once an inactive copy has expired out of TSM, it is no longer recoverable.
IBM describes it this way: (taken from page 13 of the IBM Tivoli Storage Manager for AIX: Administrator's Guide version 6.3)
The standard backup method that Tivoli Storage Manager uses is called progressive incremental backup. It is a unique and efficient method for backup. See “Progressive incremental backups” on page 13.
Progressive incremental backups
The terms differential and incremental are often used to describe backups. The standard method of backup used by Tivoli Storage Manager is progressive incremental.
The terms differential and incremental have the following meanings:
- A differential backup backs up files that have changed since the last full backup.
- If a file changes after the full backup, the changed file is backed up again by every subsequent differential backup.
- All files are backed up at the next full backup.
- An incremental backup backs up only files that have changed since the last backup, whether that backup was a full backup or another incremental backup.
- If a file changes after the full backup, the changed file is backed up only by the next incremental backup, not by all subsequent incremental backups.
- If a file has not changed since the last backup, the file is not backed up.
Tivoli Storage Manager takes incremental backup one step further. After the initial full backup of a client, no additional full backups are necessary because the server, using its database, keeps track of whether files need to be backed up. Only files that change are backed up, and then entire files are backed up, so that the server does not need to reference base versions of the files. This means savings in resources, including the network and storage.
If you choose, you can force full backup by using the selective backup function of a client in addition to the incremental backup function. You can also choose to use adaptive subfile backup, in which the server stores the base file (the complete initial backup of the file) and subsequent subfiles (the changed parts) that depend on the base file.