Quantum DLTtape DLT 7000, Product Manual

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SCSI Commands
5-164 Quantum DLT 7000 Tape System
SPACE Command (11h) (continued)
Table 5–80 SPACE Command Data - Field Descriptions (continued)
Field Name Description
Count When spacing over blocks or marks, the Count field is interpreted as follows:
A positive value N causes forward motion over N blocks or marks. The tape is
logically positioned after the Nth block or mark on the EOM or EOP side.
A value of 0 causes no change in logical position.
A negative value -N (two’s complement notation) causes reverse movement over
N blocks or marks. The tape is logically positioned on the BOM or BOP side of the
Nth block or mark.
When spacing to EOD, the Count field is ignored. Forward movement occurs until
the drive encounters EOD. The position is such that a subsequent WRITE command
would append data after the last object that has been written to tape before EOD.
When executing SPACE, the tape drive implements the following hierarchy:
Highest
BOM/P or EOM/P
EOD
Filemarks
Lowest
Blocks
Note that a “space sequential filemarks” is a space to the first occurrence of n
filemarks written sequentially.
A SPACE command in the form “SPACE N blocks” will halt with GOOD status
after the Nth block, or with CHECK CONDITION status on any occurrence of
Filemark, EOD, BOM/P, or EOM/P. A command “SPACE N Filemarks” will halt
on the Nth Filemark with GOOD status on any occurrence of EOD, BOM/P, or
EOM/P.
Depending on the size of blocks, read ahead data in the buffer allows some
spacing requests to be satisfied without actual tape movement.