IBM Ultrastar 36LZX, Технические характеристики

Страница: 227 / 249

11.9 Segmented Caching
11.9.1 Overview
Segmented Caching divides the data buffer into several smaller buffers. Each buffer is used as Read/
Write/Read-Ahead buffer.
11.9.2 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
Ÿ
RCD is 0, and
Ÿ
Read(6), Read extended(10), Read Verify, and Write and Verify is received, and
Ÿ
The consecutive LBA of the requested LBA is not available in 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 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.
11.10 Reselection Timeout
If reselection fails, it will be retried one or more times depending on the drive model. Please see the
individual drive specifications for the number of retries allowed.
11.11 Single Initiator Selection
For single initiator systems it is not an error to have only the target ID bit present during selection.
Disconnection is not allowed for Single Initiator Selection with only one ID bit present during selection.
The initiator must not send an Identify message with the disconnect permission bit(6) on.
11.12 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 initiator's 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.
Ultrastar 36LZX hard disk drive specifications
227