The scope of an incremental backup can be further reduced by what is called a partial backup.
Partial backup is not possible for UPAM files with BLOCK-CONTROL-INFO=*NO. Except for PLAM libraries. These libraries can be partially backed up because the PLAM access method replaces the PAM key with its own mechanism for marking modified sections in libraries.
As in the case of incremental backup, files which have not been modified since the last backup are not saved. If files have been modified, HSMS checks which parts of a file (2-Kbyte blocks, PAM pages) have been changed since the last full backup (not partial backup). Only these parts are saved.
As a result, the last partial backup and the last full backup are required to restore a partially backed up file.
If a file is entered as “Partially Saved” in the archive directory, the retention period of the last full backup for this file is checked. If the expiration date of the last full backup is older than the expiration date of the current partial backup, the retention period of the last full backup is updated. Consequently, the last full backup is assigned the same expiration date as the current partial backup, i.e. the date of the full backup does not expire before that of the associated partial backups. As a result, the full backup cannot be deleted before all the associated partial backups unless the user forces deletion by specifying FORCE-DELETE in the MODIFY-ARCHIVE statement.
Partial backups are requested by means of the statement
//BACKUP-FILES SELECT-FILES=*MODIFIED-FILES - // (PARTIAL-FILE-SAVE=*LARGE-FILES/*YES)
A partial backup partially saves either all files with LARGE=*YES or SAVED-PAGES=*MODIFIED-PAGES in their catalog entry or files of a specific size irrespective of the catalog entry. In the latter case, the size is specified by means of *YES(MINIMUM-SIZE=<integer>).
Note
File generation groups are always backed up completely to one save file with multiple backup versions. If a file generation or a file generation index is specified, a full backup of the index and of all file generations is performed.