Command Reordering; Concurrent I/O Process; Back To Back Write; Write Cache - IBM DCAS-32160 - Ultrastar 2.1 GB Hard Drive Specifications

11.4 Command reordering

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

11.5 Concurrent I/O Process

The Concurrent I/O process is that plural 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 executed concurrently.
Unlinked and untagged Request Sense and Inquiry during execution of other commands.
When CPE (Concurrent process enable) bit is 1, one of the following commands can be executed during
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" on
page 200 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 of 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 LBAs to be written without losing a
motor revolution.

11.7 Write Cache

If the WCE (Write cache enable) bit is 1, the drive returns Good Status and Command Complete message
and goes to Bus Free immediately after receiving the data of the last sector before actually writing the data
onto the media.
If the drive detects an error after it returns a Good Status, the drive sets a Diferred Error (Error Code of
sense data = 71h) and a following command will be returned with Check Condition and the Contingent
allegiance condition is established. Under the Contingent allegiance condition, all queued processes including
commands from other initiators are suspended.
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