Difference between differential, incremental backup and progressive incremental backup techniques


IBM Tivoli Storage Manager (TSM) supports different kinds of backup methods/types such as Full, Incremental, Differential, Progressive Incremental, Adaptive subfile and Image backup etc. Every TSM administrator might have been through this confusion on understanding the differences between differential, incremental and progressive incremental backup types. In this post I will try to explain the major differences with advantages and disadvantages of Full, incremental and progressive incremental backup techniques. Full & Incremental backup techniques are offered by many other backup tools but progressive incremental backup technique is one of the unique feature of IBM TSM which is not offered in other common storage backup tools.

Differences between Incremental and Progressive Incremental backup techniques

To understand these differences we should first know what is FULL Backup, because all other backups are dependent on this backup type only.

Also Read: What is System State Backup and why do we need it ?

Full Backup
  • IBM definition for FULL backup is "Every file on a given computer or file system is copied whether or not it has changed since the last backup".
  • TSM server will always take FULL backup when you run the backup for the first time on your client machine, you cannot change this. You can only use any other types of backups from second time.
  • Disadvantage of this type of backup is that all the files will get backup every-time you run FULL backup, this will cost you waste of network and storage resources.
Differential Backup
  • IBM definition for differential backup is "A differential backup backs up files that have changed since the last full backup".
  • That means, If a file changes after the full backup, the changed file is backed up again by every subsequent differential backup. Regular Full backups are needed in the regular intervals to decrease the backup size for example during weekends (Full + differential backup). 
  • For example, full backup is taken on Sunday and differential backups are taken from Monday - Saturday. If a file abc.doc is only changed on monday and doesn't change in any other days, it will still get backup on all remaining days until next Full backup is taken.
  • Disadvantage of this backup type is that because files which are changed since last Full backup will get backup in all subsequent Differential backups until the next Full backup is taken results in waste of network and storage resources.
  • One advantage of this backup type is (Full + differential backup) gives better restore performance than in a (Full + Incremental backup), because only two copies of data are required (the Full backup and the current Differential backup) during restoration of active client data.
Incremental Backup
  • IBM definition for incremental backup is "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".
  • That means, If a file changes after the full backup, the changed file is only backed up in its next incremental backup but not in all subsequent backups. If a file is not changed, then it will not be backed-up but make reference to the base file which is backed-up in previous backups (Full + incremental).
  • For example, full backup is taken on Sunday and incremental backups are taken from Monday - Saturday. If a file abc.doc is only changed on monday and doesn't change in any other days, it will get backup on Monday's incremental backup but not on other days. 
  • Advantage of this type of backup is less data and only one version of the file will be backed-up, saving network and storage resources.
  • Disadvantage is that restoring a single file might require retrieving multiple incremental backup copies as well as the full backup to recreate current (active) file state (Full + incremental), because each incremental backup will make index/reference to previous incremental/full backups.
Progressive Incremental Backup
  • This is the default backup type in Tivoli Storage Manager and it is designed with the combination of the backup benefits of the incremental approach with the restore benefits of the differential approach.
  • IBM definition for Progressive Incremental backup is "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".
  • For example, we can take Progressive incremental backup on all days without taking a Full backup. Suppose 10 out of 100 files are changed daily, only the 10 files will get backup and remaining 90 files references will be tracked by TSM database and it will  also consolidates the recovery media for better restore performance.

Advantages of Progressive Incremental backup Technique

  • A Full backup is done only once.
  • It eliminates unnecessary backups of unchanged data, offers more efficient use of storage resources by not storing redundant data and a faster recovery by not storing multiple versions of the same file.
  • Restoration processing will be fast because only the requested version of the file needs to be restored.
  • TSM automatically releases expired file space to be overwritten, this reduces administrator intervention and the chance of accidental overwrites of current data.
  • Over time, less data is moved than in (Full + Incremental) or (Full + Differential )backups, and data restoration is mediated by the database.
  • Progressive backups can also be combined with periodic full & selective backups without disturbing the current backup series, for example half-yearly or yearly.




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