Command Processing While Reserved; Priority Commands - Hitachi HUS151473VL3800 - Ultrastar 73.4 GB Hard Drive Specifications

20.1.10 Command Processing while Reserved

A logical unit is reserved after successful execution of the Reserve command. Each time a Reserve command is executed suc-
cessfully, the Target records the SCSI ID of the Initiator that made the reservation and the SCSI ID of the Initiator that is to
receive the reservation. This information is needed to determine whether subsequent commands should be permitted or if the
Reservation Conflict Status should be reported. The Initiator that made the reservation is the Initiator that issued the Reserve
command. The Initiator to receive the reservation may be either the same or a different Initiator (third-party reservation).
If the logical unit is reserved when a new command is received, the Target examines the command opcode and the SCSI ID of
the issuing Initiator to determine whether a Reservation Conflict Status should be returned based on the following rules:
If the issuing Initiator is the one that made the reservation and also the one to receive the reservation, then all com-
mands are permitted.
If the issuing Initiator is neither the one that made the reservation nor the one to receive the reservation, then
- A Request Sense or Inquiry command is permitted.
- A Release command is permitted but is ignored.
- Any other command results in a Reservation Conflict Status.
If the issuing Initiator is the one that made the reservation but is not the one to receive the reservation, then
- An Inquiry, Request Sense, Reserve, or Release command is permitted.
- Any other command results in a Reservation Conflict Status.
If the issuing Initiator is not the one that made the reservation but is the one to receive the reservation, then
- A Reserve command results in a Reservation Conflict Status.
- A Release command is permitted but is ignored.
- Any other command is permitted.
If a Reservation Conflict Status is not reported and the command is permitted, then the Target checks the next highest priority
internal condition to determine whether execution is allowed. See Section 20.1.1, "Priority of SCSI Status Byte Reporting" on
page 227.

20.2 Priority Commands

Certain SCSI commands always execute without returning a Busy Status or Reservation Conflict Status in response to the
command. These commands are
• Inquiry
• Request Sense
• Report LUNs
• Test Unit Ready
These commands do not disconnect from the SCSI bus prior to completion. They are executed prior to attempting to complete
the execution of any other pending command that has disconnected from the SCSI bus. Therefore, a second priority command
cannot be received during the execution of a priority command.
These commands are never queued whether or not the command is sent with a queue tag. However, the rule for an Incorrect
Initiator Connection still apply to priority commands. See Section 20.1.3, "Incorrect Initiator Connection" on page 228.
Hitachi Ultrastar 15K147 SCSI Hard Disk Drive Specification
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