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In the process of offering a new technology to the marketplace the
expertise of the user is often not considered. The HP Disk Array offers
RAID technology with special installation and configuration features
tailored for ease of use.
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The Need for Information Storage
The trend today is to put increasingly larger amounts of an organization’s information onto some sort of electronic media for quicker, concurrent, and geographically
independent access. The information being put on the electronic media is often
critical to the operation of the organization. This trend has created an increasing
demand for newer and better methods of electronic storage . One such electronic
storage method is data protection via RAID (redundant array of inexpensive disks)
technology.
drives. For RAID 3 it is necessary to provide two or more data disks plus an ECC
(error correcting code) disk. Data is dispersed or striped across the data disks,
with the ECC disk containing an exclusive-OR of the data from the other disks. Unlike the other RAID solutions, the data is dispersed across the disk in a byte interleave rather than the typical block interleave. With the spindles all synchronized,
the data is placed on the same cylinder, head, and sector at the same time. In the
case of RAID 3, each drive is connected to a dedicated SCSI channel, which
further ensures the performance.
RAID technology finds its roots in a series of papers referred to as the Berkeley
papers.1 Although these papers do not precisely define RAID, they do provide the
basis for the architecture of the technology. The need for disk drive redundancy
developed as the need for data integrity and system reliability became a growing
issue. For example, the consequence of a single point of failure in a disk storage
system could result in the failure of the entire computer system. This loss of productivity and often data was deemed unacceptable.
This architecture can handle any single disk failure in the chain. If a data drive
fails, data can be recovered from the failed drive by reconstructing the exclusive-OR of the remaining drives and the ECC drive. The advantage of this
scheme is that redundancy is achieved at a lower cost (compared to RAID 1). The
primary disadvantage is its I/O performance for small amounts of data. When an
application requires the transfer of large sequential files, such as graphic images
for workstations, this is the best method.
Seven RAID levels are defined. Each level specifies a different disk array configuration and data protection method, and each provides a different level of reliability
and performance. Since only a few of these configurations are practical for most
online transaction processing systems, file servers, and workstations, RAID levels
0, 1, 3, 5, and 6 are described here.
Fig. 3 shows the configuration for RAID 3. The data is striped across drives like
the RAID 0 configuration. If a failure occurs, the data is reconstructed based on
the parity data on disk 3.
RAID 0
Although it is often debated that since RAID 0 is not redundant and should therefore not be considered as a RAID mode, nearly all RAID solutions include this
mode. RAID 0 distributes the data across all the disks in the disk array configuration (see Fig. 1) Since there is no redundancy, the capacity utilization is 100 percent.
Disk 1
Disk 2
Disk 3
Stripe 1
B0
B1
B2
Stripe 2
B3
B4
B5
Stripe 3
B6
B7
B8
Fig. 1. RAID 0 configuration.
The data blocks in Fig. 1 are broken into three parts and striped across three disks.
What this means is that while data is being retrieved on disk 1, the data on disks 2 and
3 can be requested and ready to transfer sooner than if three I/O requests were made.
RAID 1
To answer the need for reliability and data integrity, system managers have often
implemented a solution in which write data is mirrored on two separate disk systems. This implementation is referred to as RAID 1. The primary advantage of
RAID 1 is its simplicity. RAID 1 provides a slight improvement in read performance
over the other implementations. However, write performance is poor because all
data is duplicated. The primary disadvantage of RAID 1 is cost, because for every
byte of storage used on a system an equal amount of storage must be provided as
a mirror. This results in a cost differential of 100% over standard nonredundant
mass storage.
In Fig. 2, the data is mirrored on each disk. In the event of a failure on disk 1, the
same data is available on disk 2.
Disk 1
Disk 2
Stripe 1
B0
B0
Stripe 2
B1
B1
Stripe 3
B2
B2
Disk 1
Disk 2
Stripe 1
B0
B1
Stripe 2
B2
B3
Stripe 3
B4
B5
Disk 3
Parity
Data
Data
Protection
Disk
Fig. 3. RAID 3 configuration.
RAID 5
RAID 5 was defined in an effort to improve the write performance of RAID 1 and
RAID 3 systems. Like RAID 3, the data blocks are distributed over the disk drives
in the system, but unlike RAID 3, the ECC data is also distributed across all the
drives (see Fig. 4). With this configuration reads and writes can be performed in
parallel.
Disk 1
Disk 2
Stripe 1
B0
B1
Stripe 2
B2
B3
B4
Stripe 3
Disk 3
B5
= Parity Data
Fig. 4. RAID 5 configuration.
RAID 6
Like RAID 0, RAID 6 is not yet accepted as a standard RAID configuration. This
configuration not only mirrors data (like RAID 1) but also stripes the information
(see Fig. 5). Because the data is striped, the performance is very similar to that
measured in a RAID 0 configuration. The penalty for use of this configuration is
that 100 percent more disk space is required.
Disk 1
Disk 2
Disk 3
Stripe 1
B0
B0
B1
Stripe 2
B1
B2
B2
Stripe 3
B3
B3
B4
Fig. 5. RAID 6 configuration.
Fig. 2. RAID 1 configuration.
RAID 3
The architecture for RAID 3 is often referred to as a parallel array because of the
parallel method that the array controller uses in reading and writing to the disk
1. D. Patterson, G. Gibson, and R. Katz, “A Case for Redundant Arrays of Inexpensive Disks
(RAID)”. ACM SIGMOD Conference Proceedings, Chicago Illinois, June 1988, pp. 109-116.
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(a)
Host PC
Operating System
Device Driver
SCSI Controller
External
Storage
System
External RAID
Hardware Controller
(b)
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• Operating system support. HP has an established customer
base on a wide range of PC server operating systems. While
some companies offer new products with limited operating
system support, the HP product requirement called for supĆ
port on all common PC server operating systems. At the
time of the first release of the product this included DOS
(for boot support), Novell NetWare, IBM OS/2, SCO UNIX,
Microsoft OS/2, and Banyan VINES.
• Capacity. Based on market research that a typical PC server
had a storage need of 1 Gbyte to 5 Gbytes, the HP Disk
Array was targeted to offer capacities ranging from 2 Gbytes
to 7 Gbytes.
• Performance. Several market research studies found
that improving I/O performance was the main reason to
purchase a disk array among most PC server users. PerforĆ
mance was therefore an important product requirement.
• Ease of use. EntryĆlevel products should have ease of use as
one of their primary attributes. This was not the case with
many of the RAID systems we examined. Thus, making it
easy for our customers to use RAID technology was an imĆ
portant requirement for the HP Disk Array product.
device number, or SCSI ID. If the device numbers are not
properly set up (e.g., two devices having the same ID numĆ
ber), the SCSI system will not work.
While all of these product requirements contributed to the
product's success, most of them can be considered to be
entryĆlevel requirements. These are the criteria necessary to
be considered a competitor in this market.
Before hard disks can be used for storing useful data, they
have to be lowĆlevel formatted. This is a process in which the
magnetic medium of the disk drive is divided into generic
fixedĆsize sectors (typically 512 bytes) where data can be
stored.
The following sections describe in more detail some of
the challenges the R&D team had to deal with to make the
HP Disk Array RAID product easy to use.
Since the PC environment is considered open (that is, hardĆ
ware and software from one PC manufacturer are expected
to operate with any other PC manufacturer's hardware and
software), products tend to be very generic when they reach
the end user. This often makes the installation and configuĆ
ration of new hardware and software more troublesome for
the user. Installing a new RAID system on a new or existing
server is no exception. In fact, it is generally more difficult
since the hardware and the RAID system must be configĆ
ured. The installation of a RAID system involves three major
steps: installing and configuring the hardware, configuring
the RAID system, and then making the added storage capacĆ
ity available to the operating system's storage pool by partiĆ
tioning and adding a file system. This process can be quite
intimidating, even for the most advanced user. Since the last
installation step is the same (at least very similar) for any
new storage, the following sections will focus on the first
two steps.
After unpacking and verifying that all the parts of a disk
array system are available, the user must first install and conĆ
figure the hard disks and then proceed to do the same for
the RAID controller.
Installing and Configuring the Hard Disks. Most RAID controllers
communicate with the hard disks via the SCSI (Small ComĆ
puter System Interface) interface protocol. SCSI is a generalĆ
purpose interface protocol that allows multiple devices such
as disks to communicate in a peerĆtoĆpeer fashion on a parĆ
allel bus interface. Each device is referenced with a
June 1995 HewlettĆPackard Journal
In addition to setting the SCSI ID, several SCSI interface paĆ
rameters must also be properly set for each device on the
SCSI bus. Among these are settings for synchronous and
asynchronous data transfer, data parity check, and maximum
data transfer speed. The SCSI devices are configured by
using hardware and software switches.
Some RAID systems provide the opportunity to loadĆbalance
the I/O transfers between multiple SCSI buses. For instance,
if the RAID system has seven hard disks and each disk can
offer a sustained transfer rate of 3 Mbytes/s, a single SCSI
8Ćbit (narrow) bus with a maximum transfer rate of 10
Mbytes/s could be saturated (7 × 3 Mbytes/s = 21 Mbytes/s
for commands and data and 0.66 × 21 Mbytes/s = 14
Mbytes/s for data assuming a typical 33% SCSI command
overhead).* Thus, the user must know the approximate charĆ
acteristics of the SCSI bus and the disk drives to distribute
the disk drives properly.
Installing and Configuring the RAID Controller. Setting up the
RAID controller is perhaps the most difficult and definitely
the most critical part of the hardware installation. During this
process the user has to power down the server, take the
cover off the server PC, and install the RAID controller into
an empty slot. If not handled properly, static electricity could
cause the entire server to malfunction after this operation.
Further, if the controller has hardware jumpers, they must be
configured before the controller is installed in the server.
Once the system is put back together again, a configuration
program, such as the EISAConfig program for an EISA system,
is run to assign host system resources to the controller (e.g.,
IRQ and BIOS addresses). Since the RAID controller has one
or more SCSI interfaces in addition to EISA, it is very comĆ
mon to configure the controller's SCSI parameters with the
EISAConfig utility.
Once the hardware is installed and configured, the RAID
system configuration can begin. To the user this is probably
the greatest challenge in the entire installation process beĆ
cause the RAID technology introduces many new terms and
concepts such as stripe set size, chunk size, and RAID level.
The user must understand how the selection of a parameter
will affect the system in terms of data protection, system
performance, and capacity utilization.
Fig. 6 shows the dependency relationships between some of
the configurable RAID system parameters. Note that Fig. 6 is
not meant as a complete picture, but only as an illustration
of the complexity involved in determining the values to asĆ
sign to these parameters. The operating system is the only
* The maximum number of devices on a narrow SCSI bus is eight, and since the disk controller
is considered to be one device, a maximum of seven hard disks can be connected to the bus.
Data Protection:
Is data protection or
redundancy required?
Hot Spare:
Does the system need a
standby disk for automatic
recovery?
Stripe Set Size:
What number of hard disks
should be used per stripe?
RAID Level:
Which RAID level best fits our
requirements?
RAID Volume Size:
What will the partition size be?
Is more than one partition required?
Performance:
How important is
performance to this volume?
Controller Cache:
If the read or write cache
should be configured, what
should it be set to?
I/O Request Type:
What is the distribution of random versus sequential I/O requests?
Operating System Type:
Operating Systems have different
specifications regarding volume and
File Allocation Table (FAT) block
sizes.
Chunk Size:
What is the largest amount of
data that can be contiguously
stored on one disk?
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B(4n–2)
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Disk 2
Disk 3
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B1
B2
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B3
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B6
B7
B8
Stripe n
B(3n–3)
B(3n–2)
B(3n–1)
(a)
Disk 1
Disk 2
Disk 3
Disk 4
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B4
B5
B6
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Stripe 3
B8
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Disk 3
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B1
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B0
Stripe 2
B3
B4
B5
B1
Stripe 3
B6
B7
B8
B2
Stripe n
B(3n–3)
B(3n–2)
B(3n–1)
Bn
Disk 1
Disk 2
Disk 3
Disk 4
(Hot Spare)
Stripe 1
B0
B1
Parity-1
Unused
Stripe 2
B2
Parity-2
B3
Unused
Stripe 3
Parity-3
B4
B5
Unused
B(2n–2)
B(2n–1)
Parity-n
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Disk 3
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Stripe Set 1
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(a)
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3)-% 3(% 42%1 #!. 0%1&/1- 3(% 01/0%1 /0%1!3)/. &/1 #!0!#)38
%70!.2)/.
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B2
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B(2n–2)
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24#( !2 )-0,% %36/1+ !.!'%-%.3 1/3/#/, !.$
(b)
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Parity-1
B1
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B5
B4
Parity-n
B(2n–1)
!),41% 1%#/5%18 ! 2823%- 6)3( ! (/3 20!1% "%&/1%
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86)5
%77)5621 -&621 %1( %7= > %6) *25 )(81(%17 55%<6
2* 1);3)16-9) -6.6 ACM SIGMOD Conference Proceedings
,-'%+2 //-12-6 81) 33 ?
!',8/=) -&621 %7= %1( %77)5621 >2: )/-%&/)
6 % COMPCON Spring 1989 !%1 5%1'-6'2 %/-*251-%
)&58%5< The RAIDBook: A Source Book for RAID Technology ",) (9-625< 2%5( -12 %.)6 -11)627% HP-UX 9.* and 10.0 for HP 9000 Series 700 and 800 computers are X/Open Company UNIX
93 branded products.
X/Open is a registered trademark, and the X device is a trademark, of X/Open Company Ltd.
in the UK and other countries.
UNIX is a registered trademark in the United States and other countries, licensed exclusively
through X/Open Company Limited.
Microsoft is a U.S. registered trademark of Microsoft Corp.
81) ):/)77?%'.%5( 2851%/