Hitachi Ultrastar 15K450 (FC-AL) Hard Disk Drive Specification
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21.6.4 Command Processing During Execution of Active I/O Process
When the target is not executing any I/O processes, a new I/O process is permitted to execute (unless execution is prevented by
another internal target condition listed in 21.6.1, “Priority of SCSI Status Byte Reporting” on page 315).
If an active I/O process exists when the target receives a new command, then the target determines if:
• the command is permitted to execute
• the command is added to the queue
• Queue Full status is to be returned
• Busy status is to be returned
If an active I/O process exists when the target receives a new command, then the target determines how the new command
should be handled based on the following rules:
• Check Condition status is returned with sense key set to Logical Unit Not Ready if:
• the startup operation or a format operation is active. See 21.6.6, “Command Processing During Startup and For-
mat Operations” on page319 for the exact conditions which cause this response.
Note: If a Unit Attention is pending when this condition exists, the sense key is set to Unit Attention rather than Logical Unit
Not Ready since Unit Attention has a higher reporting priority (see 21.6.1, “Priority of SCSI Status Byte Reporting” on
page 315).
• The command is permitted to execute if:
• the command is a priority command (see 21.7, “Priority Commands” on page 327).
• the conditions to execute concurrently are met (see 21.10, “Concurrent I/O Process” on page 328).
• The command is added to the queue if:
• any I/O process already exists at the target, and
• this is not an incorrect initiator connection.
• Queue Full status is returned if:
• the command would otherwise be added to the queue (according to the rules described above), but all slots in
the queue are full, or
• the command would otherwise be added to the queue (according to the rules described above), but all of the
available queue slots not reserved for use by another initiator are full, or
• a Format Unit command was previously queued but has not yet begun execution, or
• the target is in a Degraded Mode (see 21.6.9, “Degraded Mode” on page 320) and a Start Unit command was pre-
viously queued but has not yet begun execution.
• Busy status is never returned.