In the multiplexing operating mode, multiple ARCHIVE subtasks can simultaneously sharethe same MTC devices and magnetic tape cartridges in parallel. This ensures enhanced performance and the optimal utilization of tape devices. In addition, the magnetic tape cartridges are optimally filled.
Multiplexing is activated by specifying PARALLEL-RUNS=*MULTIPLEXING(..) in the HSMS statement BACKUP-NODE-FILES. Multiplexing can also be defined at archive level using the HSMS statements CREATE-ARCHIVE and MODIFY-ARCHIVE-ATTRIBUTES.
When restoring node files (//RESTORE-NODE-FILES), you do not need to specify an operand in order to activate multiplexing operation. ARCHIVE automatically calculates the number of required subtasks.
For more detailed information on multiplexing operation, see "Parallel and serial processing in ARCHIVE".
Examples
Below we illustrate the various ways in which users can activate multiplexing operation and how HSMS constructs its multiplexing configuration in response to these:
1st example: Backing up a single path
// BACKUP-NODE-FILES PATH-NAMES=*PATH-NAME( PATH=/<dir>,NODE-ID=<node-id>) OPERATION-CONTROL=*PAR(PARALLEL-RUNS=*MULTIPLEXING(NUMBER-OF-DEVICES= <integer 1..16>,MULTIPLEXING-FACTOR=<integer 2..14> oder *AUTOMATIC))
Multiplexing operation does not occur in this example since parallel processing is not possible. A multiplexing environment is nevertheless set up (one subtask for a tape drive) since the save file must always have “multiplexed” format. This makes it possible to continue a multiplexed tape later.
2nd example: Restoring one file or multiple files backed up using the same subtask
If only one file is to be backed up, only a single subtask can run. Consequently, it is not necessary to set up a multiplexing environment. Only one subtask is generated and this works directly with the device (as in the case of runs without multiplexing operation).
The same applies if all the files which are to be restored were backed up by the same subtask. Such files cannot be restored in parallel since they were stored sequentially on the tape.
3rd example: Restoring multiple multiplexed files (i.e. files which were backed up by multiple subtasks in parallel)
If multiple files which all belong to the same save version have been backed up by different subtasks, all the files can be restored in parallel (the files are all intermixed on the tape and the tape is not rewound during the restore operation).
ARCHIVE automatically calculates the number of required subtasks and sets up the environment necessary for a parallel restore. If 2 files are to be restored, a maximum of only 2 subtasks is required even if the save version was created with 14 subtasks. ARCHIVE always uses the smallest possible number of subtasks.
To illustrate:
File F1 is backed up by subtask T1, F2 by T2, F3 by T3 and F4 by T4
on device G1 in save file SF1, save version SV1.File F5 is backed up by T1, F6 by T2
on device G1 in the save file SF2, save version SV2.
To restore all the files, 4 subtasks are required for save version SV1 and 2 subtasks for SV2.
If PARALLEL-RUNS=1 applies, ARCHIVE generates 4 subtasks (=max(4,2)) and 1 tape drive task.
If PARALLEL-RUNS=2 applies, ARCHIVE generates 8 subtasks and 2 tape drive tasks; the same multiplexing factor is used for each of the devices since ARCHIVE does not know which tape drive will process save file SF1 or SF2. The restoration of F1, F2, F3 and F4 requires 2 subtasks (=max(2,2)). The same rule applies to the calculation of the configuration: the result is PARALLEL-RUNS x (2 subtasks + 1 tape drive).