Technical information

HGST Ultrastar C15K600 Hard Disk Drive Specification
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20.8
Segmented Caching
20.8.1
Overview
Segmented Caching divides the data buffer into several smaller buffers. Each buffer is used as Read/
Write/Read-Ahead buffer.
20.8.2
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 commands. 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.
20.9
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.
20.9.1
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.
20.9.2
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 20.1.5 "Unit Attention Condition" for more information.
20.10
Multiple Initiator Environment
20.10.1
Initiator Sense Data
Separate sense data is reserved for each I-T-L. Each sense data is maintained independent of commands from
other initiators.
20.10.2
Initiator Mode Select/Mode Sense Parameters
A single shared copy of the Mode Select/Mode Sense parameters is maintained by the drive. This includes both the
current and saved parameters.