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SDLT 600 SCSI Interface Guide Chapter 4: SCSI Commands

81-81200-02 REV A01 January 2005 4-8

4.1.5 Data Cache and Tape Write Interaction

The Quantum SDLT tape drive contains a data cache that buffers blocks and filemarks until it writes

them to tape. This section describes what happens when the drive writes, or “flushes,” those blocks to

tape. A MODE SELECT parameter allows the data cache to be disabled (unbuffered mode). In this

mode, every WRITE command causes data to be written to the tape medium before the drive returns

the STATUS byte and the TASK COMPLETE message to the host.

NOTE: Unbuffered mode is NOT recommended due to the poor performance that

may result.

The drive writes the contents of the write data cache to the tape medium under the following

circumstances:

• When the initiator issues a WRITE FILEMARKS command with the Immediate bit set to 0.

• When data has been in the cache longer than the maximum time specified by the value of the

MODE SELECT command Write Delay Time parameter (the default is 10 seconds).

• When the drive receives a non-write-type media access command.

• When a RESET condition occurs.

4.2 SCSI Command Descriptions

The SCSI command descriptions that make up the rest of this chapter contain detailed information

about each command the SDLT tape drive supports. The SCSI commands are presented in alphabetical

order. Fields common to many of the SCSI commands are not repeated for every command; instead

they are supported as shown in Table 4-5:

Table 4-5. Fields Common to Many SCSI Commands

Name of Field How Field is Supported in SCSI Commands

Logical Unit Number (LUN) LUN for the SDLT tape drive is 0.

Control All SCSI commands must have a Control byte, which is implemented as

described in SCSI Architecture Model – 3 (SAM–3).

Reserved Reserved bits, fields, bytes, and code values are set aside for future

standardization and must be set to 0. If the drive receives a command that

contains non-zero bits in a reserved field or a reserved code value, the

drive terminates the command with a CHECK CONDITION status and

the Sense Key is set to ILLEGAL REQUEST.

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