seagate ST12450W Installation guide

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Barracuda 2, 2HP Disc Drive

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

ST12450W/WD

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Product Manual, Volume 1

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Barracuda 2, 2HP Disc Drive

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

ST12450W/WD

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Product Manual, Volume 1

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○

Seagate®, Seagate Technology®, and the Seagate logo are registered trademarks of Seagate Technology, Inc. Barracuda™ is a trademark of Seagate Technology, Inc. Other product names are registered trademarks or trademarks of their owners.

Seagate reserves the right to change, without notice, product offerings or specifications. No part of this publication may be reproduced in any form without written permission from Seagate Technology, Inc.

Revision status summary sheet

Sheets

Revision Date Writer/Engineer Affected

A (preliminary-01) 1/25/94 L. Newman/J. Olson All A (preliminary-02) 2/15/94 L. Newman/J. Olson All A 3/29/94 L. Newman/J. Olson All B 10/3/95 L. Newman/J. Olson 8, 14, 15, 19, 24, 25,

26, 27, 29, 30, 32, 43, 44, 49, 51, 52, 53, 55, 60

Note. Product Manual 83328900 is Volume 1 of a two-volume

document with the SCSI interface information in the

SCSI-2 Interface Product Manual,

Part Number 77738479.

Volume 2

ST12450W/WD Product Manual, Rev. B v

Contents

1.0 Scope ................................................................................................................................. 1

2.0 Applicable standard and reference documentation ....................................................... 3

2.1 Standards ..................................................................................................................... 3

2.2 Applicable reference documents................................................................................... 3

3.0 General description........................................................................................................... 5

4.0 Standard features.............................................................................................................. 7

4.1 Performance ................................................................................................................. 7

4.1.1 Reliability............................................................................................................. 7

4.2 Unformatted and formatted capacities .......................................................................... 8

4.3 Options ......................................................................................................................... 8

4.4 Installation..................................................................................................................... 9

5.0 Performance characteristics ............................................................................................ 11

5.1 Internal drive characteristics ......................................................................................... 11

5.2 SCSI seek performance characteristics ........................................................................ 11

5.2.1 Seek time ............................................................................................................ 13

5.2.2 Format drive command execution time................................................................ 13

5.3 General performance characteristics ............................................................................ 14

5.4 Start/stop time............................................................................................................... 15

5.5 Prefetch/multi-segmented cache control ....................................................................... 15

5.5.1 Adaptive read lookahead..................................................................................... 16

5.6 Caching write data ........................................................................................................ 17

5.7 Synchronized spindle operation .................................................................................... 17

6.0 Reliability specifications................................................................................................... 21

6.1 Error rates..................................................................................................................... 21

6.1.1 Environmental interference.................................................................................. 21

6.1.2 Write errors ......................................................................................................... 21

6.1.3 Seek errors ......................................................................................................... 22

6.2 Reliability and service ................................................................................................... 22

6.2.1 Mean time between failures (MTBF).................................................................... 22

6.2.2 Air flow ................................................................................................................ 22

6.2.3 Preventive maintenance...................................................................................... 26

6.2.4 Service life........................................................................................................... 26

6.2.5 Service philosophy .............................................................................................. 26

6.2.6 Installation ........................................................................................................... 26

6.2.7 Service tools........................................................................................................ 26

6.2.8 Hot plugging Barracuda 2, 2HP disc drives ......................................................... 26

vi ST12450W/WD Product Manual, Rev. B

7.0 Physical/electrical specifications ................................................................................... 29

7.1 AC power requirements ............................................................................................ 29

7.2 DC power requirements ............................................................................................ 29

7.2.1 Conducted noise immunity ............................................................................ 30

7.2.2 Power sequencing ......................................................................................... 30

7.2.3 12V current profile ......................................................................................... 31

7.3 Heat/power dissipation.............................................................................................. 32

7.4 Environmental limits.................................................................................................. 32

7.4.1 Temperature.................................................................................................. 32

7.4.2 Relative humidity ........................................................................................... 32

7.4.3 Effective altitude (sea level) ........................................................................... 32

7.4.4 Shock and vibration ....................................................................................... 33

7.4.5 Air cleanliness ............................................................................................... 34

7.4.6 Acoustics ....................................................................................................... 34

7.5 Electromagnetic compatibility.................................................................................... 34

7.6 Mechanical specifications ......................................................................................... 35

7.6.1 Drive orientation ............................................................................................ 36

7.6.2 Cooling .......................................................................................................... 36

8.0 Media characteristics ...................................................................................................... 37

8.1 Media description...................................................................................................... 37

9.0 Defect and error management ........................................................................................ 39

9.1 Defects and errors .................................................................................................... 39

10.0 Option/configuration headers......................................................................................... 41

10.1 Drive ID/option select headers .................................................................................. 41

10.2 Synchronized spindles interface ............................................................................... 44

10.2.1 Electrical description...................................................................................... 44

10.3 Grounding ................................................................................................................. 45

11.0 Interface requirements .................................................................................................... 47

11.1 General description ................................................................................................... 47

11.2 SCSI interface messages supported......................................................................... 47

11.3 SCSI interface commands supported........................................................................ 48

11.3.1 Inquiry data.................................................................................................... 50

11.3.2 Mode sense data ........................................................................................... 50

11.4 SCSI bus conditions and miscellaneous features supported ..................................... 53

11.5 Synchronous data transfer ........................................................................................ 54

11.5.1 Synchronous data transfer periods supported .............................................. 54

11.5.2 REQ/ACK offset............................................................................................ 54

11.6 DC cable and connector ........................................................................................... 54

ST12450W/WD Product Manual, Rev. B vii

11.7 SCSI physical interface ............................................................................................. 55

11.7.1 Physical characteristics ................................................................................ 55

11.7.2 Connector requirements ............................................................................... 56

11.7.3 Electrical description ..................................................................................... 56

11.8 SCSI wide physical interface..................................................................................... 61

Index .......................................................................................................................................... 67

Figures

Figure 1. Barracuda 2, 2HP disc drive ...................................................................................... 1

Figure 2. Barracuda 2, 2HP disc drive (exploded view) ............................................................ 5

Figure 3. OEM interruptible thermal calibration implementation ................................................ 12

Figure 4. Synchronized drive interconnect diagram .................................................................. 17

Figure 5. Synchronized reference signal characteristics........................................................... 18

Figure 6. Air flow....................................................................................................................... 23

Figure 7. GYHX temperature measurement locations .............................................................. 24

Figure 8. RYGX temperature measurement locations .............................................................. 25

Figure 9. Temperature measurement location .......................................................................... 25

Figure 10. Typical Barracuda 2, 2HP drive +12V current profile ................................................. 31

Figure 11. Mounting configuration dimensions............................................................................ 35

Figure 12. Recommended mounting........................................................................................... 36

Figure 13. Barracuda 2, 2HP drive physical interface ................................................................. 41

Figure 14. Barracuda drive ID and option select headers ........................................................... 42

Figure 15. SCSI reference index signal driver/receiver combination........................................... 44

Figure 16. J5 configuration select header specification............................................................... 45

Figure 17. Barracuda 2, 2HP physical interface.......................................................................... 55

Figure 18. Single-ended transmitters and receivers.................................................................... 57

Figure 19. Typical differential I//O line transmitter/receiver and terminators................................ 59

Figure 20. Wide SCSI device connector ..................................................................................... 61

ST12450W/WD Product Manual, Rev. B 1

1.0 Scope

This manual describes Seagate Technol ogy®, Inc. Barracuda™ 2, 2HP (2 head parallel) disc drives.

Barracuda drives support the Small Computer System Interface-2 (SCSI-

2) as described in the ANSI SCSI and SCSI-2 interface specifications to the extent described in this manual. This manual defines the performance characteristics of the Barracuda 2, 2HP drives. The

Pr oduct Manual

characteristics of this and other families of Seagate drives.

(part number 77738479) describes general SCSI interface

SCSI-2 Interface

Figure 1. Barracuda 2, 2HP disc drive

ST12450W/WD Product Manual, Rev. B 3

2.0 Applicable standard and reference documentation

Seagate takes all reasonable steps to insure that its products are certifiable to currently accepted standards. Typical applications of these disc drives include customer packaging and subsystem design.

Safety agencies conditionally certify component parts, such as the Barracuda disc drive, based on their final acceptability in the end-use product. The subsystem designer is responsible for meeting these conditions of acceptability in obtaining safety/regulatory agency compliance in their end-use product and certifying where required by law.

2.1 Standards

The Barracuda disc drive is a UL recognized component per UL1950, CSA certified to CSA C22.2 No. 950-M89, and VDE certified to VDE 0805 and EN60950.

The Barracuda disc drive is supplied as a component part. It is the responsibility of the subsystem designer to meet EMC/regulatory requirements. Engineering test characterizations of radiated emissions are available from the Seagate safety department.

2.2 Applicable reference documents

Barracuda 2, 2HP Installation Guide

Seagate part number: 83328890

SCSI-2 Interface Product Manual (volume 2)

Seagate part number: 77738479

ANSI small computer system interface (SCSI) documents

ANSI X3.131-1986 (SCSI-1) X3T9.2/86-109 Rev. 10H (SCSI-2) X3T9.2/91-010 Rev. 10 (SCSI-3) Parallel Interface

In case of conflict between this document and any referenced document, this document takes precedence.

ST12450W/WD Product Manual, Rev. B 5

3.0 General description

Barracuda drives are low-cost, high-performance, random-access storage devices designed to meet the needs of the original equipment manufacturer (OEM) marketplace.

The Barracuda 2, 2HP drive has two read/write channels on the circuit board. The SCSI-2 controller chip is modified to process data for two read/write channels at once. This technology effectively doubles the internal data transfer rate when compared to a conventional Barracuda 2 disc drive.

The Barracuda drive’s interface supports multiple initiators, disconnect/ reconnect, self-configuring host software, and automatic features that relieve the host from knowing the physical characteristics of the targets (logical block addressing is used).

The head and disc assembly (HDA) is environmentally sealed at the factory. Air circulates within the HDA through a non-replaceable filter to maintain a contamination-free HDA environment.

Refer to Figure 2 for an exploded view of the drive. This exploded view is for information only—never disassemble the HDA and do not attempt to service items in the sealed enclosure (heads, media, actuator, etc.) as this requires special facilities. The drive contains no replaceable parts. Opening the HDA voids your warranty.

Figure 2. Barracuda 2, 2HP disc drive (exploded view)

6 ST12450W/WD Product Manual, Rev. B

Barracuda drives use a dedicated landing zone at the innermost radius of the media to eliminate the possibility of destroying or degrading data by landing in the data zone. The drive automatically goes to the landing zone when the power is removed.

An automatic shipping lock prevents potential damage to the heads and discs that results from movement during shipping and handling. The shipping lock disengages when power is applied to the drive and the head load process begins.

Barracuda drives decode track 0 location data from the dedicated servo surface to eliminate mechanical transducer adjustments and relate d r eliability concerns.

A high-performance actuator assembly with a low-inertia, balanced, patented, straight-arm design provides excellent performance with minimal power dissipation.

ST12450W/WD Product Manual, Rev. B 7

4.0 Standard features

Barracuda 2, 2HP drives have the following standard features:

• 2 head parallel technology

• Integrated SCSI controller

• Single-ended or differential SCSI drivers and receivers

• Asynchronous and synchronous data-transfer protocols

• Firmware downloadable using a SCSI interface

• Selectable sector size from 180 to 4,096 bytes per sector

• Programmable sector-reallocation scheme

• Flawed sector reallocation at format time

• Programmable auto-write and auto-read reallocation

• Reallocation of defects on command (post format)

• 96-bit Reed-Solomon error-correction code

• Sealed head and disc assembly (HDA)

• No preventive maintenance or adjustments required

• Dedicated head-landing zone

• Automatic shipping lock

• Automatic thermal compensation

• Embedded Grey Code track address to eliminate seek errors

• Self-diagnostics performed at power-on

• 1:1 interleave

• Zone bit recording (ZBR)

• Vertical, horizontal, or top-down mounting

• Dynamic spindle brake

• Permanently mounted IC terminators (ST12450W models only)

• 1,024 Kbyte data buffer (see Section 5.5)

4.1 Performance

• Simultaneously processes data for two read/write channels (heads)

• Programmable multi-segmentable cache buffer

• 7,200 RPM spindle; average latency = 4.17 msec

• Command queuing of up to 64 commands

• Background processing of queue

• Supports start and stop commands

• Provides synchronized spindle capability

• Low audible noise for office environment

• Low power consumption

4.1.1 Reliability

• 500,000 hour MTBF

• Adaptive seek velocity; improved seek performance

• LSI circuitry

• Balanced low-mass rotary voice-coil actuator

8 ST12450W/WD Product Manual, Rev. B

4.2 Unformatted and Formatted Capacities

Standard OEM models are formatted to have 512-byte sectors. ST12450 drives have nine (9) spare sectors per cylinder and one (1) spare

cylinder per unit.

Formatted Unformatted

1,849 Mbytes 2,437 Mbytes

Users having the necessar y equipment may modify t he data block size before issuing a format command to obtain different formatted capacities. User-available capacity also depends on the spare reallocation scheme selected . See the Mode Sel ect command and the Fo rmat comm and in the

SCSI-2 Interface Product Manual

(part number 77738479).

4.3 Options

The capacity shown in Section 4.2 is normally provided. Other capacities can be ord ered depend ing on the spar ing scheme an d sector size requested.

The following options are incorporated at the time of production or are available separately.

• Front panel (green lens), part number 70869751 The standard front panel is black plastic. You may order other colors. Each

panel has a single rectangular green LED indicator lens that, when glowing, indicates the drive is selected.

•

Barracuda 2, 2HP Installation Guide,

This manual provides basic information on the care and handling of Winchester disc peripherals in general. Some drive installation information is included to assist those persons not familiar with the produc t. It also includes basic information to assist in obtaining service for the drive.

• Single-unit shipping pack kit The drive is shipped in bulk packaging to provide maximum protection

against transit damage. Units shipped individually require additional prot ection as provided by the single-unit shi pping pack.

• Adapter accessory frame kit, part number 75790701 This kit contains the frame to allow a 3.5-inch drive to be mounted in

a 5.25-inch drive bay. It includes mounting hardware, front panel with a green lens, an LED with cable that connects to the remote LED connector, and installation instructions.

part number 83328890

ST12450W/WD Product Manual, Rev. B 9

4.4 Installation

For option jumper locations and definitions refer to Figure 14. Drive default mode parameters are not normally needed for installation. Refer to Section 11.3.2 for default mode parameters if you need them.

• Ensure that the SCSI ID of the drive is not the same as the host adapter.

• If multiple devices are on the bus, set the drive’s SCSI ID to one that is not presently used by other devices on the bus.

• If the drive is the only device on the bus, attach it to the end of the SCSI bus cable. Internal termination is available on single-ended (ST12450W) drives by enabling this feature with a jumper (see Figure 14). External terminators are required for differential (ST12450WD) drives. These external terminators m ust be prov ided by the user , sy stems int egrator , or host equipment manufacturer.

• If you attach the drive to a bus that contains other devices, and the new drive is not attached to the end of the bus, remove termination from the new drive.

• Set all approp riat e optio n jump ers prio r to ap plying po we r to th e driv e. If you change j umpers after applying power, recycle the drive’s power to make th e new settings eff ecti v e .

Formatting

• It is not necessary to low-level format this drive. The drive is shipped from the factory low-level formatted in 512-byte sectors.

• Reformat the drive if: a. You select a different sector size.

b. You select a different spare-sector allocation scheme.

ST12450W/WD Product Manual, Rev. B 11

5.0 Performance characteristics

This section provides performance-related characteristics and features of Barracuda 2, 2HP drives.

5.1 Internal drive characteristics

Drive capacity, Mbytes unformatted 2,437 Read/write data heads, maximum (physical) 18 Read/write data heads, maximum (logical) 9 Bytes per track, average bytes 99,536 Bytes per surface, Mbytes unformatted 271.4 Cylinders/tracks per surface, user accessible 2,710 Tracks per inch 3,047 Bits per inch 52,187 Servo heads 1 Internal data rate per physical head, Mbits/sec, 34.3 to 56.5

variable with zone Internal data rate per logical head, Mbits/sec, 68.6 to 113.0

variable with zone Disc rotation speed 7,200 ± 0.5% Average rotational latency, msec 4.17

5.2 SCSI seek performance characteristics

All performance charact eristics assume that automatic adaptive thermal compensation is not in process when the drive receives the SCSI command. Automatic adaptive thermal compensation will not interrupt an active SCSI comma n d. If ad apt i ve ther mal c ompensa ti on i s in pr oce ss whe n a SCSI command is received, the command is queued until the compensation for the specific head bei ng compensated completes. When compensation completes for the specific head being compensated, the first queued SCSI command executes. When execution of the first queued command is complete, the drive continues the compensation for the remaining head(s).

The above procedure continues until compensation for all heads has completed, or until 10 mi nutes have elapsed. The drive initiates an automatic adaptive thermal compensation cycle once on power-up before completing its initialization sequence and then once after 1 minute from the end of initialization. After this, the drive initiates an automatic adaptive thermal compensation cycle approximately once every 10 minutes. Noninterruptible automatic thermal compensation occurs at other times but should be transparent to the user (e.g., during format, Re-zer o command, at spindle-up, during read-error recovery, and during reassign-block functions) . You can use the R e-ze ro command t o r eset the t hermal compens ation timer back to its start so that the host knows when the interruption for thermal compensation will occur.

12 ST12450W/WD Product Manual, Rev. B

Refer to Section 11.8 and to the

SCSI-2 Interface Product Manual

number 77738479) for additional timing details.

Start

Has

10 min.

timer

expired?

Set

10 minute

timer

Is

SCSI

command in

progress?

Complete Command

Yes

All

heads

calibrated?

Select

next

head

Yes

received a SCSI

command?

calibrated?

TCAL

head

Has

target

All

heads

Yes

(part

Has

target

received a SCSI

command?

Yes

Target

completes

SCSI

command

Seek to

TCAL cyl and select next head

Seek to

TCAL cyl and select head zero

Figure 3. OEM interruptible thermal calibration implementation

ST12450W/WD Product Manual, Rev. B 13

5.2.1 Seek time

Drive level Drive including Barracuda controller overhead msec without disconnect

read write read write

Average typical* 8.0 9.0 9.0 10.0 Single track typical* 0.6 0.9 1.6 1.9 Full stroke typical* 17 19 18.0 20.0

*Typical seek values ar e measured under nominal condi tions o f temper a-

ture, voltage, and horizontal orientation on a representative sample of drives.

5.2.2 Format drive command execution time

≥≥

for

≥ 512-byte sectors

≥≥

Maximum (with verify) 18 minutes Maximum (without verify) 12 minutes

14 ST12450W/WD Product Manual, Rev. B

5.3 General performance characteristics

Minimum sector interleave 1 to 1 Data buffer to/from disc media (512-byte sector)

Data transfer rate (≤ 1 sector) Minimum 8.56 Mbytes/sec*

Maximum 14.11 Mbytes/sec* Data transfer rate (< 1 track) Minimum 6.2 Mbytes/sec*

Maximum 10.3 Mbytes/sec*

SCSI interface data

Asynchronous transfer rate Asynchronous* Maximum instantaneous 10.0 Mbytes/sec**

Sector sizes

Default 512-byte data blocks

Variable in even-sector sizes 180 to 4,096 bytes Synchronous transfer rate, 2 bytes wide 5.0 to 20 Mbytes/sec Read/write consecutive sectors on a track Yes Flaw reallocation performance impact Negligible

(for flaws reallocated using the spare sectors per track reallocation scheme)

Flaw reallocation performance impact Negligible (for flaws reallocated using the spare sectors per cylinder reallocation scheme)

Flaw reallocation performance impact 35 msec (typical) (for flaws reallocated using the spare tracks per volume reallocation scheme)

Overhead time for head switch 0.7 msec Overhead time for one-track cylinder switch 1.6 msec (typical) Average rotational latency 4.17 msec * Rate measured from the start of the first sector transfer to or from the host.

**Assumes s ystem abi lity to support 5.0 Mbytes/s ec, 2 bytes wid e, and no

cable loss.

ST12450W/WD Product Manual, Rev. B 15

5.4 Start/stop time

Disabling the Motor Start option causes the drive to become ready within 30 seconds after DC power is applied at nominal voltage. This means the motor starts as soon as power is applied. If a recoverable error condition is detected during the start sequence, the drive will execute a recovery procedure and may cause the time to become ready to exceed 30 seconds. During this time the drive responds to some commands over the SCSI interface. Stop time is less than 30 seconds (maximum) from removal of DC power.

Enabling the Motor Start option causes the internal controller to accept the commands listed in the than 3 seconds after applying DC power. After receiving the Motor Start command, the drive becomes ready for normal operations within 30 seconds (excluding error recovery procedure). The Motor Start command can also be used to command the drive to stop the spindle (see the Start/Stop command information in the

There is no power control switch on the drive.

SCSI-2 Interface Product Manual

SCSI-2 Interface Product Manual).

(77738479) less

5.5 Prefetch/multi-segmented cache control

The drive provides a prefetch/multi-segmented cache algorithm, which in man y c ase s e nhan ces sys tem perf orma nce . T o sele ct th is f ea ture, the host sends the Mode Select command with the proper values in the applicable bytes in page 08h (see the prefetch and cache operation enabled.

SCSI-2 Interface Product Manual).

Default is

Of the 1,024 Kbytes physical buffer space, approximately 960 Kbytes can be used as a cache. The cache can be divided into logical segments from which data is read and to which data is written.

The drive keeps track of the logical block addresses of the data stored in each segment of the cache. If the cache is enabled (see RCD bit, Table 5.2.1-27 in the

SCSI-2 Interface Product Manual),

read command is retrieved from the cache before any disc access is initiated. Data in contiguous logical blocks immediately beyond that requested by the Read command can be retrieved and stored in the cache for immediate transfer to the initiator on subsequent read commands. This is referred to as the prefetch operation. Since data that is prefetched may replace data already in the cache segment, an initiator can limit the amount of prefetch data to optimize system performance. The drive never prefetches more sectors than the number specified in bytes 8 and 9 of Mode page 08h (see the

SCSI-2 Interface Product Manual).

Kbytes of the buffer are used as a circular buffer for read/writes, with no prefetch operation and no segmented cache operation.

The following is a simplified description of the prefetch/cache operation: Case A. A read command is received and the first logical block is already in

the cache.

1. The drive transfers to the initiator the first logical block requested plus all subsequent contiguous logical blocks that are already in the cache. This data may be in multiple segments.

data requested by the host with a

If the cache is not enabled, 960

16 ST12450W/WD Product Manual, Rev. B

2. When a re quest ed logica l block is rea ched that is not in an y s egment, the d rive fetches i t and any r emaining re quested logi cal block a ddresses from the d isc and p uts them in a se gment of the cache. The drive transfers the remaining requested logical blocks from the cache t o the initiator in accordance with the “buffer-full” ratio specification given in Mode Select Disconnect/Reconnect parameters, page 02h (see the

SCSI-2 Int erfac e Pr o duct Man ual

3. The drive prefetches additional logical blocks contiguous to those transferred in step 2 above and stores them in the segment. The drive stops filling the segment when the maximum prefetch v alue has been transferred (s ee the

Case B. A read command is received and the first logical block address requested is not in any segment of the cache.

1. The drive fetches the requested logical blocks from the disc and transfers them into a segment, then from there to the initiator in accordance with the “buffer-full” ratio specification given in Mode Select Disconnect/Reconnect parameters, page 02h (see the

face Pr oduct Manual

2. The drive prefetches additional logical blocks contiguous to those transferred in Case A, step 2 above and stores them in the segment. The drive stops filling the segment when the maximum prefetch value has been transferred.

SCSI-2 Inter fac e Pr oduct Manual

SCSI-2 Inter-

During a prefetch, the drive crosses a cylinder boundary to fetch data only if the Discontinuity (DISC ) bit is set to 1 in bit 4 of byte 2 of the Mode Select parameters page 8h. Default is zero for bit 4 (see the

Pro duct Manual

Each cache segment is ac tually a self-c ontained circular buffer whose length is an integer number of sectors. The wrap-around capability of the individual segments greatly enhance s the cache’s overall performance, allowing a wide range of user-selectable configurations including a pu re prefetch strategy. The drive supports operation of any integer number of segments from 1 to 1 6. Divide the 976, 896 bytes in the buffer by the number of segments to get the segment size . Default is 3 segments. (See the

2 Interfac e Pr o duct Manual.

)

5.5.1 Adaptive read lookahead

Read lookahead causes the drive to continue reading data from the disc following a normal read I/O until the read-ahead parameters are satisfied (prefetch). If subsequent I/O requests can be satisfied from the prefetched data in the data buffer, there is a significant improvement in performance since a disc access is eliminated. If subsequent I/O requests cannot be satisfied from the prefetched data, there is a reduction in performance due to prefetch overhead.

The adaptive read lookahead feature suspends the prefetch operation if three subsequent read I/O requests are not satisfied from the prefetched data. This improves performance because unnecessary prefetches are avoided.

SCSI-2 Interface

SCSI-

Prefetch is reinstated if an I/O request is sequential to a previous I/O request.

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