The recommendations presented can be used individually and independently of each other. They are a,ways beneficial in all situations.
Reducing the backup time through parallel I/Os (increased I/O load!)
Backing up multiple volumes in parallel under their own FDDRL subtasks. The degree of parallelization can be specified with the FDDRL parameter TASK-LIMIT.
Setting the task priority for the FDDRL subtasks
If an FDDRL run is to be executed as quickly as possible and without taking into account the response time behavior during BS2000 operation while the workload is low, the TASK-LIMIT parameter of the job class should have the value of the available tape devices (at most 16) in order to achieve a good throughput through the h igh degree of parallelization. The FDDRL parameter RUN-PRIORITY should be set to “high”.
If an FDDRL run is to be executed during normal BS2000 operation without impairing the performance of the main application, the TASK-LIMIT parameter of the job class should be set relatively low to take into account the other applications. In the event of multiple parallel runs a noticeable worsening of the response time behavior of the other applications must be expected. The RUN-PRIORITY parameter should be set to “low”.
Increasing throughput of disks and tapes
Using large tape blocks increases the performance and tape utilization. By default, large tape blocks are written when saving to MTC with FDDRL.
Multiplexing with PAV (/390 servers)
FDDRL generates asynchronous I/O operations when reading from disk (DUMP) or writing to disk (RELOAD). These are executed in parallel when the storage system’s Parallel Access Volume (PAV, 390 server) function is used together with a suitable RAID level (see "RAID levels and their performance").
Multiplexing with multiple disks
In the case of very high-speed tape storage systems, FDDRL can read blocks from up to 4 disks in parallel and write alternately to tape. This process makes sense particularly when disk processing is slower than tape processing. Depending on the tape storage systems available and the size of the tape volume used, the following backup strategies can be employed:
Large number of tape drives and small tape volumes
With ETERNUS CS a large number of tape drives are as a rule available. The volume size is configurable and should be configured in relation to the disk volume size to avoid too high tape VSN consumption or frequent tape changes occurring. Here it is recommendable to use the FDDRL statement //DUMP-PUBSET ..., SAVE-ENTITY=*SINGLE-DISK to back up pubsets.
One tape set is written for each disk volume. This ensures that maximum performance is achieved. The tape and disk sizes should be coordinated to achieve a good tape occupancy level.
Few tape drives and large tape volumes
When LTO drives are used, the following FDDRL statement is recommended: //DUMP-PUBSET ..., SAVE-ENTITY=*DISK-SET(NUMBER-OF-DISK-SETS=n)
Multiplexing enables multiple disk volumes to be backed up on a small number of tape volumes and tape drives. The number of drives used in parallel can be set using the NUMBER-OF-DISK-SETS parameter. The TASK-LIMIT must be coordinated with this using //MODIFY-FDDRL-PARAMETERS. Since multiple disk volumes are backed up on one tape set, large tape volumes can be used effectively.
The creation of disk sets using //DUMP-PUBSET also has the advantage that complete and consistent restoration of the pubset is possible with //RELOAD-PUBSET. If disk sets are not used, the pubset must be restored with //RELOAD-DISK. In this case the system administrator has the task of guaranteeing the completeness and consistency of the various restored volumes.
In addition to the operand values mentioned, TAPE-FORMAT=*STD should also be set (//MODIFY-FDDRL-PARAMETERS). FDDRL then determines the ideal tape format itself. A different format specification is only provided for data interchange with systems using earlier FDDRL versions.