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Quantum DLT 7000 Tape System 5-69
MODE SELECT (6) / (10) Command (15h) / (55h) (continued)
Table 5–37 MODE SELECT Mode Parameter Header - Field Descriptions
Field Name Description
Media Type This field is ignored by the MODE SELECT command.
Buffered Mode Default = 1. The drive implements immediate reporting on WRITE commands
through its buffered mode. With Buffered Mode set to 1, the drive reports
GOOD status on WRITE commands as soon as the data block has been
transferred to the buffer. If this field = 0, then the drive does not report GOOD
status on WRITE commands until the data blocks have been written to tape.
When Buffered Mode is not used, the tape drive suffers significant performance
degradation, and possible capacity, depending on tape format, block size, and
compression. When writing 2.6 or 6.0 GB formats and block size that is a
multiple of 4 Kbytes, there is no capacity loss. Likewise, when using the 10 or
20 GB format with compression disabled and block size a multiple of 8 Kbytes,
there is no capacity loss.
When writing 10 or 20 GB format with compression enabled and Buffered
Mode disabled, some capacity loss can occur. The block packing feature is
essentially disabled by turning off Buffered Mode.
If Buffered Mode is set to a number greater than 1, the command is rejected
with CHECK CONDITION, sense key of ILLEGAL REQUEST.
Speed The tape drive supports only its default speed.
Block Descriptor
Length
This field specifies the length in bytes of all the block descriptors. Since the
drive only allows one block description, the value must be either 0 or 8. A value
of 0 indicates no block description is included; a value of 8 indicates a block
descriptor is present and precedes the mode page data. Any other value other
than 0 or 8 causes a CHECK CONDITION status with sense key of ILLEGAL
REQUEST to be returned.