Compaq 108164-003 - ProLiant - 800 Quick reference guide

WHITEPAPER
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ECG025.0997
[September 1997]
Prepared By
Microsoft Windows NT
Integration Team
Compaq Computer
Corporation
CONTENTS
Disk Subsystem
Overview
.....................
3
Disk-Related
Performance
Characteristics
............
4
Like Drive
Scalability
.................
17
Like Capacity
Scalability
.................
19
Disk Controller
Scalability
.................
24
Performance
Measurement Tools
...
27
Preventing Data
Loss while
Maintaining
Performance
..............
28
Disk Subsystem
Summary of
Findings
....................
29
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.)
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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.
Copyright ©1997 Compaq Computer Corporation. All rights reserved. Printed in the
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

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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

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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.

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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.

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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.

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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.

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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
1
A
2B
3C
4D
5
E
6
F
G
H7
8
5
3
1
7A
C
E
G
6
4
2
8B
D
F
H
Disk Head
Disk Head Disk Head
Example 1
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.

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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
1
A
2B
3C
4
D
5
E
6
F
G
H7
8
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

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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
1
1
3
4
2
2
Host Bus (Memory)
transfer rate
3
Host Bus transfers to/from
ProLiant 5000
4
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.