Hitachi Ultrastar 15K450 (FC-AL) Hard Disk Drive Specification
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21.13 Segmented Caching

21.13.1 Overview

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

21.13.2 Read Ahead

The Read Ahead function consists of reading data that the Initiator has not yet requested to the drive buffer. This function is
intended to improve performance for an initiator that frequently accesses sequential data with successive SCSI read com-
mands. The Read Ahead function works when RCD (the read cache disable) bit of read cache page (page 08h) is set to zero.
The drive initiates the Read ahead function when the following conditions exist:
• RCD is 0
• Read, Verify and Write and Verify is received.
• The consecutive LBA of the requested LBA is not available in the buffer
If SCSI reset or target reset message is received, all contents of segmented buffer is flushed.
Even if an error occurs during the Read ahead, the error will not be reported to the Initiator. The data read before the error
occurred will be stored as valid data by the Read Ahead function.
21.14 Multiple Initiator Systems
This section describes how the target behaves in a multiple initiator system. Up to 64 initiators may be supported at any one
time.

21.14.1 Sense Data

A separate sense data area is reserved for each initiator. Each area is maintained independently. This allows a command from
one initiator to complete with a CHECK CONDITION status and generate sense data without being affected by a subsequent
command from a different initiator. There is no requirement for the first initiator to send a REQUEST SENSE command to
retrieve the Sense Data prior to the execution of a command from a different initiator.

21.14.2 Mode Pages

A single set of Mode pages is maintained. This includes both current and saved parameters. If a MODE SELECT command is
executed that updates the current parameters, a unit attention condition is generated for all initiators except the one that issued
the command. See 21.6.5, “Unit Attention Condition” on page 317 for more information.
21.15 Enclosure Services
Enclosure Services allow the Host Computer System to access information held by the Enclosure Microcontroller using the
same industry standard interface as the enclosed devices.
This disk drive supports the ESI (Enclosure Services Interface) for this purpose, as described in the Functional Specification.
The drive sets the EncServ bit to 1b in the Standard INQUIRY Data indicating that it supports enclosure services. This indi-
cates that the Host may use the SEND DIAGNOSTIC and RECEIVE DIAGNOSTIC RESULTS commands to access informa-
tion held by the Enclosure. This information is in the form of 'Diagnostic Pages'.
Diagnostic Pages 01h to 0Fh are reserved for Enclosure Services.
There are two distinct forms of Enclosure Services. The EncServ bit does not specify which form the Target and Enclosure are
capable of (if any). The Host can discover this by using a RECEIVE DIAGNOSTIC RESULTS command to request Diag-
notic Page 00h.
When requested to communicate with the enclosure via the SEND and RECEIVE DIAGNOSTIC commands, the drive goes