A full backup saves all selected files completely (except for migrated files), regardless of whether or not they have been changed since the last backup.
Backups performed by the HSMS administrator include data of online migrated files if the appropriate backup method has been selected. The backup includes the catalog entries from the catalog and the data from the migration level. It is also possible to limit the backup of migrated files to their catalog entries.
For files on a Net-Storage volume of NETSTOR volume type you can also choose whether the entire file (default) or only the catalog entry is to be backed up (SAVE-OPTIONS= *PAR(SAVE-NET-STOR-DATA=*YES/NO) operand), while Net-Storage files residing on volumes of type NETVOL are backed up entirely irrespectively what is specified in the SAVE-NET-STOR-DATA operand.
An incremental backup does not save all specified files. If HSMS detects that the appropriate archive already contains the current contents of a file, the file is not saved again. Instead it is registered in the directory as Cataloged-Not-Saved (CNS). As a result, only those files whose contents were modified or newly created since the last backup are saved. Incremental backups can therefore result in a considerable reduction of the time and storage space required for a system backup. They are carried out automatically by HSMS unless otherwise specified. Node files on Net-Storage are always fully backed up – also in the case of incremental backups.
If a file is entered as Cataloged-Not-Saved (CNS) in the archive directory, the retention period of the last full backup of this file is checked. If the expiration date of the last full backup is older than the expiration date of the current incremental 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 incremental backup, i.e. the date of the full backup does not expire before that of the associated incremental backup. As a result, the full backup cannot be deleted before all the associated incremental backups unless the user forces deletion by specifying FORCE-DELETE in the MODIFY-ARCHIVE statement.
If only the catalogue entries of the migrated files have been saved with the last full backup (SAVE-DATA=*STD/*S0)), the data of the migrated files also have to be saved (SAVE-DATA=*S1-S0/*S2-S1-S0), so that in case of a crash at migration level, the data can be recovered. Details on S1/S2 crash troubleshooting can be found here: section "Using RESTORE on migrated files".
Full backup offline
A full backup may be initiated either for storage level S0 (FROM=*S0), for previous backups stored in an archive (FROM=*ONLY-LATEST-BACKUPS) or for both (FROM=*LATEST-BACKUPS-OR-S0).
In the latter case (FROM=*LATEST-BACKUPS-OR-S0), the backup is produced from the copies in the save file of all files currently stored in the archive rather than from the contents of the files at storage level S0. As a result, time requirements for a full backup at storage level S0 are similar to those for an incremental backup, since the procedure is as follows: first all modified files of storage level S0 are saved. Once this step has been completed, storage level S0 is again available. The backup is then continued, with files being copied from tape to tape.
In a full backup with FROM=*ONLY-LATEST-BACKUPS, the complete backup is performed offline. No preparatory online steps are necessary at storage level S0 because the last incremental backup is taken as a reference. All files which are entered in this incremental backup (fully or as CNS), are backed up from their last save and combined into a single new full backup. But if this last incremental backup contains files which were only partially saved (partial backup), these files are skipped because they cannot be restored offline.
A full backup of migrated files with FROM=*ONLY-LATEST-BACKUPS is not permitted. Migrated files which were marked as MIGF in the last backup remain as MIGFs in a pure full backup offline.
To be able to guarantee a complete offline backup which includes storage levels S0, S1 and S2, the previous last backup run must have been started with the specification SAVE-DATA=*S2-S1-S0 and must have been performed without partial backup.
Special aspects
A full backup with the operand LATEST-BACKUPS-OR-S0 proceeds in exactly the same way as any other full backup, except for the handling of the metadata (catalog entry) of unmodified files. Files that have not been changed are not taken from the S0 storage level, instead they are taken from the last full or incremental backup (including their metadata).
S0 catalogued files are the basis of the file volume to be saved, therefore, no data on S0 should be deleted; this also applies to offline backups.
A full backup of S0 or of the most recent backup in the archive always has to start a new save file; it is not possible to continue an existing save file.
Up to 255 partial backups of a file can be performed between any two full backups of this file. The 256th backup is automatically executed as a full backup for this file even if otherwise specified in the HSMS statement.
The type of backup is controlled via the SELECT-FILES operand of the BACKUP-FILES statement. Specifying SELECT-FILES=*MODIFIED-FILES results in an incremental backup, while SELECT-FILES=*ALL-FILES causes a full backup to be carried out.
The type of full backup can be specified via the FROM operand. The default value *S0 causes the files of storage level S0 to be saved. If the value *LATEST-BACKUPS-OR-S0 is specified, the backup of CNS files contained in the archive is copied.
Making a full backup by means of incremental backups works like this:
The system catalog is checked to determine the status of the files.
All modified files are backed up from storage level S0.
All unmodified files are backed up from tape.