Compaq 166207-B21 - Smart Array 5302/32 RAID Controller, 166207-B21 5302/32 Technology Brief
Page 1
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: TechCom@hp.com
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1
Page 2
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
Page 3
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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drives in the array.
or more
Page 4
TC020604TB
TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding
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4
Requires a minimum of one drive.
Requires a minimum of two drives.
Requires a minimum of four drives.
Requires a minimum of three drives.
Pn represents one set of parity.
Compaq RAID Advanced Data Guarding
Requires a minimum of four drives.
Pn and Qn represent two sets of parity.
Table 1. Summary of RAID Technologies for Large Arrays
RAID LEVELS Function/Applications Limitations
RAID 0
RAID 1
RAID 1+0
RAID 5
Files are split up and
striped across separate
disk drives.
Image Editing • Video
Production • Pre-Press
Applications
Mirroring - Identical
data stored on two
drives, high fault
tolerance, very good
performance.
Accounting • Payroll •
Financial
Implemented as a stripe
of mirrored disks.
Database applications
requiring high
performance and fault
tolerance, but willing to
sacrifice storage
efficiency.
One set of parity data is
distributed across all
drives. Protects against
the failure of any one
drive in an array.
Transaction processing •
File and application
servers • ERP • Internet
and Intranet servers
Two sets of parity data
are distributed across all
drives. Protects against
the failure of two drives
in an array. Provides
higher fault tolerance
than RAID 5.
For 24x7 applications
that require a higher
level of fault tolerance
than RAID 5.
No fault tolerance
and highly
vulnerable to
failure. The entire
array will fail if
one drive fails.
Requires 50% of
capacity to be
dedicated to fault
protection.
Doubles the
number of drives
required.
Relatively low
fault tolerance. It
can only
withstand the loss
of one drive
without total array
failure so it is
highly risky for
large arrays.
Lower write
performance than
other RAID
methods.
Page 5
N
ote: RAID ADG is
A
standard with the fourchannel Compaq Smart
rray 5300 (SA 5304/256)
Controllers and is sold as
an option for the twochannel Smart Array models
(SA 5302/128). RAID ADG
requires a minimum of
64 MB read/write cache.
TC020604TB
TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding
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RAID ADG PROVIDES OPTIMUM FAULT TOLERANCE
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Often, the terms “reliability” and “fault tolerance” are used interchangeably in describing RAID
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schemes; however, there is a distinction between them. Reliability refers to the likelihood that an
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individual drive or drive array will continue to function without experiencing a failure. Reliability
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is typically measured over some period of time. Although reliability is a very important issue for
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customers, it is a function of the reliability of the individual array components rather than the
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RAID technology implemented.
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Fault tolerance, on the other hand, is the ability to withstand and recover from a failure despite
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disk drive reliability issues. Fault tolerance is provided by some sort of redundancy—mirroring,
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parity, or a combination of both—and it is typically measured by the number of drives that can fail
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without bringing down the entire array. The fault tolerance of various RAID levels is as follows:
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• RAID 0 has no fault tolerance because it uses no type of redundancy. The array will fail if one
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5
physical drive fails.
• With RAID 1 or 1+0, the maximum number of hard drives that can fail without failure of the
array is n/2, assuming none of the failed drives are mirrored to each other. In practice, logical
drive failure will usually occur before this maximum number is reached, since the probability
of a newly failed drive not being mirrored to a previously failed drive becomes increasingly
small as the number of failed drives increases. A RAID 1+0 array will fail if a drive and its
mirrored drive fail, but the probability of this decreases as the number of mirrored pairs
increase.
• RAID 5 can withstand the failure of one physical drive. The array will fail if two drives fail
simultaneously.
• RAID ADG can withstand the failure of two physical drives. Three hard drives must fail
before the entire array will fail. RAID ADG also protects against the loss of data if a drive
fails and a defect occurs in a single sector of another drive. This is important if data is being
rebuilt after a drive failure and a media defect occurs in one of the good drives.
Although RAID 1 and RAID 1+0 provide a higher level of fault tolerance than RAID 5, this
protection comes at a very high price because 50 percent of the drives are dedicated to fault
protection. For RAID 5 configurations, we recommend using no more than 14 physical drives be
used per array due to the increased likelihood of drive array failure with more hard drives. For
arrays with more than 14 drives, we recommend RAID ADG due to its fault tolerance and storage
efficiency. RAID ADG supports the use of up to 56 physical drives per array because that is the
maximum number of hard drives that can be physically connected to a four-channel Compaq
Smart Array Controller.
Figure 1 shows the relative probability of logical drive failure for different RAID settings and
different logical drive sizes, assuming no online spares are present. RAID ADG is less likely to
experience a logical drive failure than RAID 0, 5, and 1+0. An online spare (hot spare) can be
added to any of the fault-tolerant RAID levels to further decrease the probability of logical drive
failure. As soon as drive failure occurs, missing data can be automatically rebuilt from parity data
onto the online spare. Without an online spare there is a chance that more hard drives will fail
before the failed drive is replaced, leading to failure of the complete drive array and consequent
loss of data. This is less likely with RAID ADG than with RAID 5, since RAID ADG can sustain
failure of two drives. RAID ADG supports online spare drives and Online RAID Level Migration
from RAID 1 or RAID 5
1
For more information about online spare drives and Online RAID Level Migration from RAID 1
or RAID 5, please refer to the Advanced Data Guarding for the Smart Array 5300 Controller
Reference Guide, part number 188270-001 at
.1
www.compaq.com
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Page 6
TC020604TB
TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding
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Figure 1. Probability of drive array failure for different RAID levels. RAID ADG is less likely to experience a
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complete drive array failure than RAID 0, 1+0, and 5.
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Page 7
TC020604TB
TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding
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RAID ADG IS A COST-EFFECTIVE SOLUTION
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The cost effectiveness of each RAID solution is a balance between the total cost of the array and
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its usable capacity. While the total cost includes all the drives in the array, the usable capacity
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includes only the drives that store non-redundant (not parity or mirrored) data. One way to
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evaluate cost effectiveness is to compare the cost per gigabyte of usable capacity of various RAID
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levels. Another useful way to evaluate cost effectiveness is to compare storage efficiency—the
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usable capacity divided by the total of capacity of all the drives.
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An important factor to note is that the usable capacity of any RAID array is limited by the size of
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the smallest hard drive in the array; the extra capacity on larger drives goes unused. For example,
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an array with four drives (40 GB, 60 GB, 60 GB, and 60 GB) would have a usable capacity of 4 x
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40 GB, or 160 GB. To maximize storage efficiency, all RAID array drives should have the same
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capacity. If drives with different capacities are attached to the same controller, it is possible to
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create multiple arrays that contain only drives of the same capacity.
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Table 2 lists the storage efficiencies of the various RAID levels. The storage efficiency of RAID 1
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and RAID 1+0 is constant, but the storage efficiency of RAID 5 and RAID ADG varies with the
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number of drives. The number of parity drives in RAID 5 and RAID ADG schemes is fixed (one
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parity drive for RAID 5 and two parity drives for RAID ADG), so their storage efficiency
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increases with the number of drives.
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As shown in Table 2, RAID 1 and 1+0 have the lowest storage efficiency at 50 percent; therefore,
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they are less cost-effective solutions for large arrays. A comparison of the storage efficiencies of
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RAID 5 and RAID ADG requires closer scrutiny. For a given number of drives, RAID 5 will have
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higher storage efficiency than RAID ADG, although this difference shrinks as the number of
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drives increases. The storage efficiency of a RAID 5 array varies from 67 percent for three drives
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to 93 percent for 14 drives (the maximum recommended by Compaq). The storage efficiency of
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RAID ADG varies from 50 percent for four drives to 96 percent for a maximum of 56 drives. A
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maximum of 56 drives can be physically connected to a four-channel controller using four external
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Compaq storage systems.
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The bottom line is that RAID 1 and 1+0 have very low storage efficiency. RAID 5 and RAID
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ADG have much higher storage efficiencies, but the level of efficiency depends on the number of
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drives in the array.
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Usable Capacity
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(C= capacity of smallest
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drive; n= number of drives)
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Minimum number of drives 2 4 3 4
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Maximum recommended
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number of drives*
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Storage efficiency from
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minimum to maximum
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recommended no. of drives**
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*Compaq recommends not exceeding these maximum figures (excluding any allowable online spares) when configuring
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a drive array, due to the increased likelihood of drive array failure if more hard drives are added.
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**The value for storage efficiency is calculated assuming all drives in the array have the same capacity with no online
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spares.
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7
Table 2. Summary of RAID Array Storage Efficiency
RAID 1 RAID 1+0 RAID 5 Advanced Data
C*(n/2) C*(n/2) C*(n-1) C*(n-2)
N/A N/A 14 N/A
50% 50% 67% to %93 50% to 96%
*
Guarding
Page 8
TC020604TB
TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding
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PERFORMANCE CONSIDERATIONS
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The key to RAID performance is parallelism—the ability to access multiple disks simultaneously.
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This parallelism allows data to be written to or read from a RAID array faster than would be
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possible with a single drive. Analyzing RAID performance can be very complicated because
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several factors must be considered (sequential versus random reads and writes, block size, data
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transfer rate, etc).
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The performance of a RAID array can be subdivided into read performance and write
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performance, which vary based on the RAID level. For example, in RAID 1 (mirroring), the data
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and redundant data must be written to two separate drives. The data is normally read from one
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drive, so the read performance is much faster than the write performance; however, data can be
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read from both drives to increase the read performance. RAID 0 uses striping to improve
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performance by splitting up files into smaller portions (stripes) and distributing them to multiple
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hard disks; however, RAID 0 has no fault tolerance. RAID 5 and RAID ADG also use striping, but
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their write performance is significantly affected by the multiple reads and writes needed to perform
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the parity calculations prior to updating the array. The write performance of RAID ADG is less
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than that of RAID 5 because RAID ADG has dual parity overhead. The read performance of
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RAID 5 and RAID ADG is very good and may be improved by tweaking the stripe size.
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In the final analysis, RAID array performance boundaries are largely predetermined by the
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intended application. Applications that require a large capacity array, yet have a relatively low
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ratio of writes to reads (ERP, transaction processing, and web servers) may benefit from striping
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with parity—RAID 5 or RAID ADG. On the other hand, applications that have a much higher
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ratio of writes to reads (file servers, database servers, and media development), may benefit from a
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RAID 1+0 array; but eventually cost becomes an issue as capacity requirements grow. The
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following section will help guide customers through this complicated decision-making process.
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CHOOSING A RAID LEVEL
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To choose the optimum RAID level for data protection in large arrays, customers should consider
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a variety of factors, including:
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Fault tolerance (based on availability requirements)
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Cost effectiveness (based on storage efficiency or cost per gigabyte of usable capacity)
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Performance (based on application requirements)
Customers can use the decision chart in Table 3 on the next page to determine which RAID level
provides the best solution. For example, if cost effectiveness is of primary importance and fault
tolerance is of secondary importance, or vice versa, the best choice is RAID ADG.
Page 9
TC020604TB
TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding
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9
Table 3: Important Factors in Choosing the Optimum RAID Level for Large Arrays
Most Important Secondary Importance RAID Level Choice
Fault Tolerance → RAID ADG Cost Effectiveness
Performance → RAID 5
(RAID 0 if fault tolerance is
not needed)
Cost Effectiveness → RAID ADG Fault Tolerance
Performance → RAID 1+0
Performance
Cost Effectiveness → RAID 5
(RAID 0 if fault tolerance is
not needed)
Fault Tolerance → RAID 1+0
S
UMMARY
Compaq RAID ADG provides an advanced level of data protection for customers who need a
higher level of fault tolerance than RAID 5 and a lower implementation cost than RAID 1. RAID
ADG is best implemented when customers need to protect enterprise data at a lower cost than
RAID 1 arrays, and when performance is not an overriding factor.
RAID ADG is supported on the Compaq Smart Array 5300 Controller and can effectively protect
an array of up to 56 total drives. A RAID ADG array can tolerate up to two simultaneous drive
failures without downtime or data loss. RAID ADG supports Online Spare Drives and Online
RAID Level Migration from RAID 1 or RAID 5.
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