Incremental Backup

After an image backup of the disk is made, incremental backups can be performed periodically. An incremental backup contains only the files created since the image backup. The backup date associated with each file is examined to determine if it is newly created. Incremental backups are usually smaller (because fewer files are involved) and therefore are performed faster than an image backup. There is no concept of UNIX backup levels in OpenVMS.

Two types of incremental backups are supported: cumulative and differential. A cumulative backup includes all changes since the image backup and is performed with this command:

     $ BACKUP/VERIFY/IGNORE=INTERLOCK -     _$ DUA0:[000000...]*.*;*/SINCE=BACKUP -     _$ MUA400:CUM.SAV/REWIND/LABEL=CUMULA 

This command states (on the second line) that all of DUA0: is to be searched for files created since the last full backup. Only those files are copied to MUA400:. Every time the cumulative backup runs, the save set grows, because the number of files created since the last image backup undoubtedly increases.

The differential backup command is similar; only one qualifier is added to the first line. The /RECORD specifies that the backup date of those files that are backed up are to be recorded so they are not backed up again. Hence, the command changes to this:

    $ BACKUP/VERIFY/RECORD/IGNORE=INTERLOCK -     _$ DUA0:[000000...]*.*;*/SINCE=BACKUP -     _$ MUA400:DIFF.SAV/REWIND/LABEL=DIFFER 

Successive differential backups only capture files created since the last differential backup, and the save set size is relatively constant each time it is run.