Command Reordering; Concurrent I/O Process; Back To Back Write - IBM Ultrastar 36LZX Specifications

4. Queue Tag number does not affect the order of execution.
5. If an initiator issues a command with a queue tag which is the same as the current I/O process or
queued I/O process. The target returns Incorrect Initiator connection.
6. A series of linked commands are a single I/O process and are assigned the queue tag established
in the initial selection. A command received with a Head-of-Queue tag message shall not suspend
a series of linked commands for which the target has begun execution.
7. If DQue is changed to 1 while queued commands exist, all queued commands for the all initiators
will be aborted. All future commands received from any initiator with a queue tag will be processed
as untagged commands with a message reject message being returned immediately after the qtag
is received by the target.

11.4 Command reordering

Command reordering function is supported under tagged command queuing enabled (DQue = 0). The re-
order feature reorders Read/Write commands in order to minimize seek time between commands. This
function will improve total throughput of the drive.

11.5 Concurrent I/O Process

The Concurrent I/O process when multiple I/O processes are active (not queued) on the same logical
unit at the same time. The target may start the data phase of an I/O process while another I/O process is
not completed. The following I/O processes are allowed to execute concurrently:
Ÿ Unlinked and untagged Request Sense and Inquiry during execution of other commands.
Ÿ When CPE (Concurrent process enable) bit is one, one of the following commands can be executed
while another one or the same one of the following commands is being executed, if those are
untagged or simple tagged commands.
- Read(6), Read extend(10)
- Write(6), Write extend(10)
When an I/O process ends in Check Condition Status, the drive enters the Contingent Allegiance
Condition and other queued I/O processes from all initiators on the same logical unit will not reconnect
and will not complete the execution until the sense data is cleared. See 11.16, "Contingent allegiance
Condition," for details. If an I/O process (P-1) encounters an error while another I/O process (P-2) is
active, the drive returns Check Condition to P-1 and P-2. The drive may continue P-2 until its convenient
point to suspend, but may not send a Status. After the initiator clears the Contingent Allegiance
condition, the drive will resume or terminate P-2 according to QErr bit of Control mode page.
If the drive gets an error on P-2 before suspending the execution of P-2, it will keep the sense data
separately from the sense data for P-1. The sense data for P-2 will be set after the Contingent Allegiance
condition caused by P-1 is cleared.

11.6 Back to Back Write

Back to Back Write allows plural write commands requesting sequential LBA's to be written without
losing a motor revolution.
Ultrastar 36LZX hard disk drive specifications
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