Quantum 10K II - Manual (Page 306)

Feature Descriptions
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives 6-5
6.9.4 Data Sector Reallocation Error Protection
In any SCSI disk drive, bad blocks may be reallocated. However, a power failure or
unrecoverable data could threaten data integrity during a block reallocation.
The reallocation and defect list storage algorithms prevent a reallocation from being
lost due to a power failure. Once a reallocation starts, the information about the
data block to be moved is stored on the media. As the reallocation progresses,
checkpoint information about the reallocation (such as the new destination and the
data in transit) is recorded. If the reallocation is interrupted, the process can
automatically continue from the last checkpoint without data loss.
If a block reallocation is performed on unrecoverable data (rare), a Bad Data Mark is
set in the new location. Thus, there is no chance of undetected bad data being
generated from a reallocation. Bad block reallocation is discussed in section 6.16.3.
6.9.5 Data Verification
Data can be verified in two ways. First, the integrity of data in selected areas of the
media can be checked using the VERIFY command. Secondly, the WRITE AND
VERIFY command writes host-supplied data and then verifies that it was written
correctly.
The Verify Error Recovery Page of the MODE SELECT COMMAND controls the
drive’s response to error conditions that arise during the VERIFY command and
during the verify operation of the WRITE AND VERIFY command.
6.10 TAGGED COMMAND QUEUING
The drive supports all three SCSI-defined queue tag messages. When queuing is
enabled (default condition), the drive can accept any of the following:
• Simple - specifies that the command is to be placed in the drive’s command task
set (queue). If several commands are present, the drive may reorder them to
increase overall throughput.
• Head of Queue - directs the drive to place the command at the beginning of the
queue, to be executed next. Consecutive commands with Head of Queue
specified are executed in last-in-first-out (LIFO) order. Queue depth is 64.
• Ordered - specifies that commands in the drive’s task set are to be executed in
the order received (FIFO).
The DQue bit (disable queuing) of the Control Mode page can be used to disable
tagged command queuing.