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I upgraded to SCO OpenServer 5 and BackupEDGE 1.1, and can't use block sizes over 256.
edge BackupEDGE OpenServer OSR5 tape block factor
Product Release(s)
Problem Description
Under Unix 3.2v4.2 I could use large block factors. I recently upgraded to SCO OpenServer 5 and BackupEDGE 1.1. Now I can't use block factors above 256.

SCO OpenServer 5 defaults to "Variable" Block Mode with many tape drives, especially DAT/DDS drives. In variable block mode the maximum block factor is controlled by the CTBUFSIZE kernel parameter. Its default value of 128 [1k] blocks means the maximum available BackupEDGE block factor is 256. Remember that BackupEDGE measures its blocking factor in 512-byte blocks, so the maximum blocking factor will always be twice CTBUFSIZE.

To increase the BackupEDGE block factor above 256 it is first necessary to reconfigure the CTBUFSIZE kernel paramater to [desired_block_size] /2. For instance, to use a BackupEDGE block size of 768 you must set CTBUFSIZE to 384 or higher, then relink the kernel and reboot.

	  NOTE 1:  The OpenServer 5 configuration program will display
                   warnings if you attempt to set CTBUFSIZE above 256.
                   These may be safely ignored.

NOTE 2: Make ABSOLUTELY SURE you perform a MASTER BACKUP and BIT-LEVEL Verification before trusting your data to the new block size. Some tape drives may fail to return errors at larger block sizes, but in fact incorrect data is written. The BIT-LEVEL Verify will ensure that you can use large block factors safely.

NOTE 3: To use a BackupEDGE block factor of 1024 or higher it will be necessary to increase the kernel parameter SHMMAX beyond its default of 524288, then relink and reboot.

NOTE 4: BackupEDGE double buffering does an excellent job of keeping the tape drive queue full in most instances. It is rare that using a block factor above 256 along with double buffering will result in significantly increased backup speeds on most systems.

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Last Updated - 2022/01/03