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ECG025.0997

[September 1997]

Prepared By

Microsoft Windows NT

Integration Team

Compaq Computer

Corporation

CONTENTS

Disk Subsystem

Overview

.....................

Disk-Related

Performance

Characteristics

............

Like Drive

Scalability

.................

Like Capacity

Scalability

.................

Disk Controller

Scalability

.................

Performance

Measurement Tools

...

Preventing Data

Loss while

Maintaining

Performance

..............

Disk Subsystem

Summary of

Findings

....................

Disk Subsystem Performance and

Scalability

In today's networking environments, the disk subsystem is a key element in determining

overall system performance. The goal of this paper is to provide informative test results

and performance-related information for various disk subsystems, to assist systems

engineers and network administrators in making decisions on disk subsystem installation,

optimization, and configuration.

This white paper also provides information on using Fault Tolerance to prevent data loss,

while maintaining system performance. Finally, this paper provides a section discussing

the advantages and disadvantages of RAID technology.

WHITE PAPER (cont.)

ECG025.0997

NOTICE

The information in this publication is subject to change without notice.

COMPAQ COMPUTER CORPORATION SHALL NOT BE LIABLE FOR TECHNICAL

OR EDITORIAL ERRORS OR OMISSIONS CONTAINED HEREIN, NOR FOR

INCIDENTAL OR CONSEQUENTIAL DAMAGES RESULTING FROM THE

FURNISHING, PERFORMANCE, OR USE OF THIS MATERIAL.

This publication does not constitute an endorsement of the product or products that were

tested. The configuration or configurations tested or described may or may not be the

only available solution. This test is not a determination of product quality or correctness,

nor does it ensure compliance with any federal, state or local requirements. Compaq

does not warrant products other than its own strictly as stated in Compaq product

warranties.

Product names mentioned herein may be trademarks and/or registered trademarks of

their respective companies.

Compaq, Contura, Deskpro, Fastart, Compaq Insight Manager, LTE, PageMarq,

Systempro, Systempro/LT, ProLiant, TwinTray, ROMPaq, LicensePaq, QVision, SLT,

ProLinea, SmartStart, NetFlex, DirectPlus, QuickFind, RemotePaq, BackPaq, TechPaq,

SpeedPaq, QuickBack, PaqFax, Presario, SilentCool, CompaqCare (design), Aero,

SmartStation, MiniStation, and PaqRap, registered United States Patent and Trademark

Office.

Netelligent, Armada, Cruiser, Concerto, QuickChoice, ProSignia, Systempro/XL, Net1,

LTE Elite, Vocalyst, PageMate, SoftPaq, FirstPaq, SolutionPaq, EasyPoint, EZ Help,

MaxLight, MultiLock, QuickBlank, QuickLock, UltraView, Innovate logo, Wonder Tools

logo in black/white and color, and Compaq PC Card Solution logo are trademarks and/or

service marks of Compaq Computer Corporation.

Other product names mentioned herein may be trademarks and/or registered trademarks

of their respective companies.

U.S.A.

Microsoft, Windows, Windows NT, Windows NT Server and Workstation, Microsoft SQL

Server for Windows NT are trademarks and/or registered trademarks of Microsoft

Corporation.

Disk Subsystem Performance and Scalability

First Edition (September 1997)

Document Number: ECG025.0997

WHITE PAPER (cont.)

ECG025.0997

DISK SUBSYSTEM OVERVIEW

Key components of the disk subsystem can play a major part of overall system

performance. Identifying potential bottlenecks within your disk subsystem is crucial. In

this white paper we identify and discuss in detail disk-related performance characteristics

that can help you understand how latency, average seek time, transfer rates and file

system or disk controller caching can affect your disk subsystem performance. Once we

discuss all of the disk measurement terms, we use those definitions to address

performance issues in each scalability section of this document. The different scalability

sections discussed are as follows:

•Like Drive (similar hard drive scalability)

•Like Capacity (similar drive capacity scalability)

•Disk Controller (multiple controller scalability)

This document provides disk subsystem recommendations, based on testing in the

Integration Test Lab of hardware and software products from Compaq and other vendors.

The test environment that Compaq selected might not be the same as your environment.

Because each environment has different and unique characteristics, our results might be

different than the results you obtain in your test environment.

Test Environment

The following table describes the test environment used for the disk subsystem

performance testing. This table displays both the one and two controller test

configurations that were used in the Compaq ProLiant 5000 during testing.

Table 1:

Disk Subsystem Testing Environment

Environment Equipment Used

Server Hardware Platform Compaq ProLiant 5000

Memory 128 MB

Processors (4) P6/200 MHz 512k secondary cache

Network Interface Controllers (2) Dual 10/100TX PCI UTP Controller (4 network segments)

Disk Controllers (1 or 2) SMART-2/P Controllers

Disk Drives 2, 4, or 9 GB Fast-Wide SCSI-2 drives

Number of Drives up to fourteen 2.1, 4.3, or 9.1 GB Fast-Wide SCSI-2 drives

Boot Device (1) Fast-Wide SCSI-2 drive off the Embedded C875 controller

WHITE PAPER (cont.)

ECG025.0997

Table 1: (cont.)

Disk Subsystem Testing Environment

Environment Equipment Used

Server Software Configuration Microsoft Windows NT Server version 4.0

Service Pack 2

Compaq Support Software Diskette 1.20A

Client Configuration Compaq Deskpro 575

Netelligent 10/100 TX PCI UTP Controller

and MS-DOS

NetBench 5.0 Test Configuration Disk Mix

Work Space 15 MB

Ramp Up Time 10 seconds

Ramp Down Time 10 seconds

Test Duration 120 seconds

Delay Time 0 seconds

Think Time 0 seconds

DISK-RELATED PERFORMANCE CHARACTERISTICS

Before beginning our discussion on disk subsystem performance, Table 2 lists the general

terms used in the industry to describe the performance characteristics of disk

performance. These general terms describe characteristics that can impact system

performance, so it is important to understand the meaning of each term and how it could

affect your system.

Table 2:

Disk Performance Measurement Terms

Terms Description

Seek Time The time it takes for the disk head to move across the disk to find a

particular track on a disk.

Average Seek Time The average length of time required for the disk head to move to the

track that holds the data you want. This average length of time will

generally be the time it takes to seek half way across the disk.

WHITE PAPER (cont.)

ECG025.0997

Table 2: (cont.)

Disk Performance Measurement Terms

Terms Description

Latency The time required for the disk to spin one complete revolution.

Average Latency The time required for the disk to spin half a revolution.

Average Access Time The average length of time it takes the disk to seek to the required

track plus the amount of time it takes for the disk to spin the data

under the head. Average Access Time equals Average Seek Time

plus Latency.

Transfer Rate The speed at which the bits are being transferred through an

interface from the disk to the computer.

Concurrency The number of I/O requests that can be processed simultaneously.

RPM (Revolutions Per Minute) The measurement of the rotational speed of a disk drive on a per

minute basis.

Table 2 lists the definitions of disk-related performance characteristics. Let’s now use

those definitions in the next several sections to address how adding drives to your system

can affect performance.

Seek Time and Average Seek Time

Seek time describes the time it takes for the disk head to move across the disk to find

data on another track. The track of data you want could be adjacent to your current track

or it could be the last track on the disk. Average seek time, however, is the average

amount of time it would take the disk head to move to the track that holds the data.

Generally, this average length of time will be the same amount of time it takes to seek half

way across the disk and is usually given in milliseconds.

WHITE PAPER (cont.)

ECG025.0997

One method to decrease seek time is to distribute data across multiple drives. For

instance, the initial configuration in Figure 1 shows a single disk containing data. The

new configuration reflects the data being striped across multiple disks. This method

reduces seek time because the data is spread evenly across two drives instead of one,

thus the disk head has less distance to travel. Furthermore, this method increases data

capacity because the two disks provide twice the space to store data.

Data

Seek Time

Initial

Configuration

New

Configuration

Unused space Unused space

Data

Seek Time Seek Time

Unused space

Data

Figure 1: Average Seek Time and Capacity

You can use this same concept and apply it to many different configurations. For

example, if you currently have a two-disk configuration but you want to decrease the

average seek time yet increase the disk capacity, you can configure a striped set of disks

using four disks instead of two. This concept applies to any configuration (odd or even

number of disks) as long as you are adding more drives to your stripe set.

WHITE PAPER (cont.)

ECG025.0997

Average Latency

Manufacturers have built and continue to build hard disks that spin at designated rates. In

the early years of the personal computer (PC) industry, hard disks on the market could

spin at approximately 3600 RPMs. As the market demand for better system performance

increased, disk manufacturers responded by supplying faster spin rates for hard disks.

By producing faster spinning disks, manufacturers reduced the amount of overall access

time. Average latency directly correlates to the spin rate of the disk drive because it is, as

defined earlier in Table 2, the time required for the disk to spin half a revolution.

Therefore, this direct relationship in improving hard disk spin rates can contribute to better

system performance by reducing the average latency on a disk.

Manufacturers understand the need for better system performance and continue to

provide new and improved hard disks. With today’s hard disks spinning at 7200

revolutions per minute (RPMs) and the hard disks of tomorrow spinning at the rate of

10,000 RPMs, we can see that manufactures continue to address the issue of faster

performance. Table 3 provides a brief history on hard disks listing spin rates, disk

capacities available and approximate dates the disks were available to the market.

Table 3:

Hard Disk History

Disk Spin Rate Disk Capacity Approximate Date Used

3600 RPMs Up to 500 MB 1983 – 1991

4500 RPMs 500 MB – 4.3 GB 1991 – Present

5400 RPMs 500 MB – 6 GB 1992 – Present

7200 RPMs 1 GB – 9.1 GB 1993 - Present

10,000 RPMs 4.3 GB and 9.1 GB 1997 - Present

Now that we discussed the direct relationship between disk spin rates and system

performance, let’s examine how drive scaling can affect latency. In Figure 2 - Example 1,

the initial configuration shows the disk has to spin halfway around before the disk head

can start to retrieve data from sector 5. In the new configuration, the disk has to spin half

the distance than before to retrieve the same data. Thus, the latency time has been cut

in half.

WHITE PAPER (cont.)

ECG025.0997

However, be aware that the average latency time might not always decrease when adding

more drives to your system. For example, in Figure 2 - Example 2, the new configuration

shows that the amount of time it takes to retrieve the data from sector B is actually longer

than the initial configuration. The reason for this is that the disk has to spin half way

around to read sector B. In the initial configuration the disk only had to spin one-eighth a

revolution to read the identical data. But, keep in mind that the initial configuration for

Example 2 required both seek time and latency time.

Initial

Configuration

New

Configuration

Example 2

Disk Head

Disk Head Disk Head

Example 1

Example 2

Figure 2: Average Latency

Overall, these examples show us that in some configurations, as shown in our first

example, drive scaling would be a definite performance advantage. However, in other

configurations it is not clear if you receive a performance gain because of the components

involved, such as the combination of average seek time and average latency time used in

Figure 2 – Example 2.

When you combine these terms (average seek time and average latency time), you define

another disk measurement called average access time, which is discussed in the

upcoming section. From the information provided in this section, we know seek time plus

latency (or average access time) is a key in determining if performance is truly enhanced

in your system.

Note: Remember that the disks

used in Figure 2 are identical in

size and RAID configuration.

WHITE PAPER (cont.)

ECG025.0997

Average Access Time

Average access time is simply described as average seek time plus latency. What this

equates to is the amount of time the disk has to seek to find the data plus the time it takes

for the disk to spin under the head. For example, Figure 3 contains a disk with two tracks

of data on it. Track 1 contains data sectors 1 – 8. Track 2 contains data sectors A – H.

Thus, in our example, the disk head has to move (or seek) from the current position (track

1, sector 1) to the track you want to read (track 2, sector C).

For the purpose of our illustration, Figure 3 displays the disk head performing these

functions separately. However, in reality the disk drive performs both seek and latency

functions simultaneously.

Unused space

Data

Disk Head

Latency

Seek Time

Average Seek Time + Latency = Average Access Time

Figure 3: Average Access Time

Transfer Rates

A disk subsystem is made up of multiple hardware components that communicate by

transferring data to and from the disk(s) to a computer. The main parts of a disk

subsystem are as follows:

•Hard Disks

•SCSI Channel

•Disk Controller

•I/O Bus

•File System and Disk Controller Caching

WHITE PAPER (cont.)

ECG025.0997

In order to share information, all of the disk subsystem components have to communicate

with each other, as shown in Figure 4. The disk subsystem components communicate

with each other using hardware interfaces such as Small Computer System Interface

(SCSI) channels and Peripheral Component Interconnect (PCI) buses. These

communication highways, called channels and/or buses, communicate at different rates of

speed known as transfer rates.

Each of the disk subsystem components transfer data at different rates. It is important to

understand the different transfer rates of each component because this information helps

you identify potential performance bottlenecks within your system. For example, Figure 4

shows hard disks transferring data to the SCSI channel (bus), which transfers the

information to the disk controller, which then passes the data to the Host Bus and then on

to the server. If one hard disk transfers at 5 MB/s, the SCSI channel transfers at 40 MB/s,

the disk controller transfers at 40 MB/s and the Host Bus transfers at 540 MB/s, it is

obvious that the hard disk is the bottleneck. Therefore, by knowing the transfer rate of

each subsystem device, potential bottlenecks can be easily identified and corrected.

The key to improving system performance is focusing on how to maximize data

throughput by minimizing the amount of time the disk subsystem has to wait to receive or

send data. In the upcoming sections, we discuss how to identify performance bottlenecks

and where they could possibly occur in your disk subsystem.

PCI Bus transfers

to/from the Host Bus

More Drives

Controller transfers to/from disks via SCSI channel

Disk drive average

sustained transfer

rate

SCSI Bus

transfer rate

PCI Bus

transfer rate

Host Bus (Memory)

transfer rate

Host Bus transfers to/from

ProLiant 5000

Controller transfers

to/from PCI Bus

Dual Inline Memory

Module (DIMM)

Figure 4: Disk subsystem components transferring data.

Remember that the slowest disk

subsystem component determines

the overall throughput of the

system.

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