HP LTO 4 FC, LTO 4 SCSI manual Using the SET CAPACITY command, Command descriptor block, CDB fields

Capacity calculations are based on estimates; reported values can be subject to error in two ways:

•Random errors caused by tolerances in tape length, hub diameter, and so on.

•Systematic errors caused by ignoring system areas, and so on. They ensure the calculated capacity is actually available to the user. It is usually possible to write considerably more data than the calculated capacity.

Using the SET CAPACITY command

You can modify the capacity of a tape by changing the logical length of the tape through the SET CAPACITY command. The primary use envisaged is for testing purposes, although it may also be used in other circumstances where a shortened tape may be beneficial.

NOTE: All data currently on the tape will be lost following successful execution of this command. The command is only accepted when the media is positioned at Beginning of Media (BOM).

With WORM cartridges, the command is only accepted and executed if the cartridge has not been initialized, that is, it has never been written to. Otherwise the cartridge is rejected with CHECK CONDITION, sense key of Data Protect and additional sense of 300Ch (WORM media—overwrite attempted). TapeAlert flags 3Ch (WORM media—overwrite attempted) and 09h (write-protect) are set.

Command descriptor block

	7	6	5		4	3	2	1	0
0					Operation Code (0Bh)
1				Reserved (0)					Immed
2					Reserved (0)
3	(MSB)			Capacity Proportion Value
4									(LSB)
5					Control

CDB fields

Immed	0	Status will not be returned until the SET CAPACITY operation has completed.
	1	Status will be returned as soon as the CDB has been parsed.
Capacity	The	portion of the total volume capacity to be made available for use. The value
Proportion Value	is the numerator of a fraction with a denominator of 65,535. The resulting
	available capacity will be total-------------volume---------------------capacity-----------------------⋅-----capacity------------------------proportion-----------------------------value------------ .
		65535

Note that the LTO-4 format enforces a minimum tape length. A value that would result in a tape length below this minimum will be silently rounded up to the minimum permitted length.

HP LTO Ultrium 4 drives technical reference manual, volume 2: software integration 21 HP restricted