Performance

System backups generally involve very large quantities of data. FDDRL offers a number of functions for speeding up the transfer of data and thus reducing the time required for the saves:

• When saving a disk, FDDRL normally transfers only those areas which are marked as occupied in the F5 label of the disk. However, if the saving of the disk was preceded by an abnormally terminated BS2000 session, the whole disk is saved automatically.

• In the case of V15 and V16 tape format (see the TAPE-FORMAT operand in statement, "MODIFY-FDDRL-PARAMETERS Define execution parameters") ,FDDRL works with large blocks provided the tape devices support this (as of TAPE-C3).

• In order to speed up the disk side to the higher data transfer rate of the tape devices in the case of dump and reload, FDDRL utilizes parallel access to disk devices by means of PAV (Parallel Access Volumes). In the case of V16 tape format with disk sets, FDDRL uses multiplexing by accessing two disks at a time for a tape device. In the case of the V17 tape format with disk sets, FDDRL uses multiplexing by accessing up to four disks at a time for a tape device.

FDDRL can perform its functions under separate tasks (FDDRL subtasks). The TASK-LIMIT parameter allows you to preset the degree of parallelism (see also chapter "FDDRL requests, FDDRL jobs, FDDRL subtasks"). By means of the RUN-PRIORITY parameter, you can also specify for each FDDRL subtask the task priority within the job class.

When FDDRL is used, two different cases can be distinguished with respect to throughput and performance:

• When BS2000 is running with a small number of applications, an FDDRL run should be performed as quickly as possible and without regard for the response time behavior.

  In this case, you can set the TASK-LIMIT parameter up to the limits of the available tape devices in order to achieve a good throughput rate from the high degree of parallelism. With four or more parallel runs, however, noticeably worse response time behavior is to be expected.

  You can also set a high RUN-PRIORITY (low value).

• During normal BS2000 operation, an FDDRL run should be performed without impairing the response time or the performance.

  You must take the other applications into account when setting the TASK-LIMIT parameter; above all, sufficient tape devices must be kept free. The RUN-PRIORITY parameter must be set low (high value).

To allow efficient control of resource utilization (tape devices, CPU time, task limit) during an FDDRL run as well, you should set up a separate job class for FDDRL.

• The throughput can be optimized if one MTC controller with its own channel is available for every two disks.

• Software compression with the //DUMP-DISK and //DUMP-PUBSET statements uses a considerable amount of CPU resources. Therefore, if restrictions in response time behavior are not acceptable, you should not use software compression.

Each new tape format promises better performance than its predecessor. The improvement affects both the tape and the disk side.

As far as possible, you should therefore always use the latest tape format (set using the FDDRL parameter TAPE-FORMAT=*STD).