Externalization throughput and the number of tape cartridges used depend on the output block size. The actual block size is selected by setting and dynamically refreshing the VTGROUP parmlib attribute BackstoreBlocksize. The recommended value for this attribute is 0K specifying that CA Vtape must select the appropriate block size based on the output device type.
Note: If high density tapes (IBM 3590 or STK 9840 or equivalent) are being used in 3490 emulation, set this attribute to 256K. If 0K is used, CA Vtape will select 64KB as the block size which will be inefficient for these tape devices.
If the default block size value of 16K is being used, Externalization on high density tapes can achieve around a 6 MBs per second end-to-end transfer rate when only Primary is defined. End-to-end transfer rate includes the time needed for label processing, cataloging the data set, and waiting for the tape management system response. Externalization with Primary and Duplex tapes can achieve around a 5 MBs per second end-to-end transfer rate. Use Volume Mount Analyzer reports to determine how much data will be written to the Virtual Volumes per hour. Convert this value to megabytes per second. Divide the result by the transfer rates achieved to determine the number of physical tape drives required to support Externalization.
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