Compaq 166207-B21 - Smart Array 5302/32 RAID Controller, 166207-B21 5302/32 Technology Brief

June 2002

Compaq Computer
Corporation

Prepared by
ISS Technology
Communications

CONTENTS

Executive Summary..........1

Introduction .....................3

RAID Levels:
Functions and

Limitations.......................3

RAID ADG Provides
Optimum Fault

Tolerance .........................5

RAID ADG is a
Cost-Effective

Solution ...........................7

Performance

Considerations.................8

Choosing a RAID

level .................................8

Summary..........................9

TC020604TB

TECHNOLOGY BRIEF

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Compaq RAID Advanced Data Guarding:

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A Cost-effective, Fault-tolerant Solution

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EXECUTIVE S UMMARY

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Although most customers are not experiencing sharp increases in business revenue,

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almost all are experiencing an explosion in the volume of enterprise data they are

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accruing. As customers evaluate storage solutions for this data, they are focusing on three

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very important needs: fault tolerance, storage capacity efficiency (cost effectiveness), and

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performance. Compaq RAID Advanced Data Guarding (ADG) is a cost-effective, fault

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tolerant solution for storing large volumes of enterprise data. Its performance, like that of

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other RAID levels, depends on the nature of the application. RAID ADG is supported on

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the Compaq Smart Array 5300 Controller and the Smart Array Cluster Storage and

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Modular SAN Array 1000 controllers.

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Organizations implementing a large array should consider RAID ADG because it can

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tolerate up to two simultaneous drive failures without downtime or data loss. It can safely

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protect an array of up to 56 hard disk drives with greater fault tolerance than RAID 5 and

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with a lower implementation cost than RAID 1.

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Please direct comments regarding this communication to the ISSG Technology Communications Group at this Internet

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address: [e-mail]

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1

TC020604TB

TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding

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NOTICE

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Compaq shall not be liable for technical or editorial errors or omissions contained

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herein. The information is provided “as is” without warranty of any kind and is

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subject to change without notice. The warranties for Compaq products are set

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forth in the express limited warranty statements accompanying such products.

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Nothing herein should be construed as constituting an additional warranty.

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Compaq and the Compaq logo are trademarks of Compaq Information Technologies Group, L.P.

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in the U.S. and/or other countries.

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All other product names mentioned herein may be trademarks of their respective companies.

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2002 Compaq Information Technologies Group, L.P.

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Compaq Computer Corporation is a wholly-owned subsidiary of Hewlett-Packard Company.

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Compaq RAID Advanced Data Guarding: A Cost-effective,

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Fault-tolerant Solution

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First Edition (June 2002)

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Document Number TC020604TB

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2

R

AID - Redundant array

D

of independent disks.

Striping – The
spreading of data over
multiple disk drives to
improve performance.

ata is interleaved by
bytes or by sectors
across the drives.

TC020604TB

TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding

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INTRODUCTION

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Customers are being flooded by a rising tide of data that is the result of e-business and traditional

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applications: transaction processing, enterprise resource planning, decision analysis, etc. As the

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data level rises, customers are focusing on cost-effective storage technologies to protect the data

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they are amassing on an increasing number of disk drives. For these customers, RAID provides

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better performance and fault tolerance than storing data on separate hard drives. RAID enables a

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group of disk drives to function, from the operating system perspective, like a single physical disk

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drive.

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RAID schemes, called levels, are differentiated by the method each uses to provide fault tolerance,

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but there is no correlation between the level numbers and the degree of fault protection. RAID 1,

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1+0, 5, and Compaq RAID ADG are described in this paper because they are the levels best suited

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for arrays that store enterprise data. RAID 0 does not provide fault tolerance; however, it is

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described here in relation to RAID 1+0.

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This paper first describes the functions and limitations of various RAID levels in protecting data in

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large storage volumes. Then, it describes three of the most important factors customers should

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consider in their decision-making process: fault tolerance, cost effectiveness, and performance.

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RAID LEVELS: FUNCTIONS AND LIMITATIONS

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Customers needing to create large arrays with a high number of disk drives, or with high-capacity

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disk drives, should consider the limitations of current RAID schemes in protecting data during a

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single- or multiple-drive failure. This section will help customers distinguish between the different

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RAID levels, including RAID ADG. Table 1 on the following page summarizes these RAID

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technologies in regards to function, best-suited applications, and limitations.

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In a RAID 0 implementation, a file is split into blocks and each block is striped across all the

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drives in the array. For large files, reading this data in parallel from the separate drives is faster

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than reading the file from a single drive. However, this RAID scheme offers no fault tolerance; the

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entire array will fail if one drive fails.

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RAID 1 is a mirroring scheme that stores identical data on two sets of drives. It is used in

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applications that require very high availability. RAID 1 has high fault tolerance, but it has low

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storage efficiency because it requires twice the number of drives. RAID 1+0 is implemented as a

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striped array of mirrored drives. It is best suited for sites that need high performance and

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maximum reliability, but are willing to sacrifice storage efficiency. RAID 1+0 can withstand the

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failure of half the drives as long as no two drives in a mirrored pair fail; however, it sacrifices

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storage capacity efficiency.

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RAID 5 can be implemented on arrays of three or more drives. Parity information is calculated for

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each stripe of data and is placed on a drive other than the drive used to store the stripe of data. The

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parity information is striped across all drives in the array, and it occupies the equivalent capacity

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of one physical drive. Overall, RAID 5 provides good performance, but it can only withstand the

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loss of one drive without failure of the array.

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Compaq RAID ADG is an extension of RAID 5 for implementation on arrays of four

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drives. Data and two sets of parity information are striped across all drives in the array. The

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additional set of parity improves the fault tolerance of the array but results in lower write

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performance. The two parity sets occupy the equivalent capacity of two drives in the array. RAID

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ADG protects against the simultaneous failure of two

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3

drives in the array.

or more