HP LTO Ultrium 4 drives technical reference manual, volume 2: software integration 21
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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
CDB fields
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.
76543210
0 Operation Code (0Bh)
1 Reserved (0) Immed
2 Reserved (0)
3 (MSB)
Capacity Proportion Value
4 (LSB)
5 Control
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
Proportion Value
The portion of the total volume capacity to be made available for use. The 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
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