Segmented Caching; Overview; Read Ahead; Reselection - IBM DCAS-32160 - Ultrastar 2.1 GB Hard Drive Specifications

11.10 Segmented Caching

11.10.1 Overview

Segmented Caching divides the data buffer into several smaller buffers. Each buffer is used as
Read/Write/Read-Ahead buffer.

11.10.2 Read Ahead

The Read Ahead function consists of reading data that the Initiator has not yet requested to the file buffer.
This function is intended to improve performance for an initiator that frequently accesses sequential data
with successive SCSI read commands. The Read Ahead function works when R C D (the read cache disable)
bit of read cache page (page 08h) is set to 0.
The drive initiates the Read ahead function when
R C D is 0, and
Read(6), Read extended(10), Read Verify, and Write and Verify is received, and
The consecutive LBA of the requested LBA is not available on the buffer.
If SCSI reset or bus device reset message is received, all contents of segmented buffer is flushed.
Even if an error occurs during Read ahead, the error will not be reported to the Initiator. The data read
before the error occurs will be stored as a valid data by Read ahead.

11.11 Reselection

A reselection timeout error occurs when the target attempts to reselect an initiator and the initiator does not
respond within a Selection Timeout delay(250 mSec). If this occurs, the target releases the SCSI bus going to
the bus free phase and waits 250ms. It will then retry the reselection up to 16 times. If the initiator still does
not respond the command is terminated and the target generates sense data with a Sense Key of Aborted
Command and an Additional Sense code of Select/Reselect failure(45h). If any of the reselection attempts is
successful then the command is resumed.

11.12 Single Initiator Selection

For single initiator systems, it is not an error to have only the target ID bit present during selection. Discon-
nection is not allowed for Single Initiator Selection with only one ID bit present during selection. The initi-
ator must not send an Identify message with the disconnect permission bit(6) on.

11.13 Non-arbitrating systems

The Target cannot detect whether other SCSI devices on the SCSI bus use arbitration prior to selection. As
a consequence, the Target allows disconnect permission to be enabled by the Identify message independent
of the initiators use of arbitration prior to selection. A non-arbitrating initiator must ensure that disconnect
permission in the Identify message is disabled (bit 6=0)for proper operation.
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O E M Spec. of DCAS-34330/32160