The Question is:
What is the optimal block_size setting when
performing an image backup to a DLT IV tape
The Answer is :
Usually a multiple of 4096 blocks for DLT, 61440 being a typical choice.
If you trust the SCSI bus and the DLT hardware and particularly the DLT
CRC processing, /GROUPS=0 and /NOCRC can be reasonable -- this choice
is usually based on the perceived value of the data, and the history
with the hardware. (BACKUP with the XOR grouping and CRC enabled will
provide end-to-end verification of the data.)
Enabling compaction can help with tape usage and with the reduction of
the physical media access bandwidth -- this compression usually (but not
always) decreases the amount of data that is written to the DLT media.
Also configure the BACKUP operator username account per the process quota
settings referenced in the OpenVMS SYstem Manager's manual.