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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