Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

RAID was described in the paper "A Case for Redundant Arrays of Inexpensive

Disks (RAID)", Patterson, Gibson and Katz, Department of Electrical Engineering and

Computer Sciences, University of California at Berkeley, 1988

DRAFT -- Revision 2.0

August 22, 1996Page 4

INTRODUCTION

Disk technologies have been rapidly changing with new generation hardware available

every nine to 12 months. Computer buyers have been having trouble keeping up with

the changes let alone understanding them. RAID technology in particular has become

popular in the marketplace. RAID is an acronym for Redundant Arrays of Inexpensive

Disks and was designed as an alternative to Single Large Expensive Disks (SLED)

used on supercomputers and mainframes. Often, buyers choose RAID technology for

emotional

reasons rather than logical reasons or just because it's the latest

hot

technology

without understanding the tradeoffs to make an informed decision.

The High Availability (HA) environment seems to be where much of the controversy

exists. Computer buyers want a disk technology that provides the best availability for

the least cost, in another words price/availability. Like price/performance,

price/availability has many tradeoffs that need to be evaluated. The highest availability

will not cost the least amount of money!

If asked, a computer buyer usually says that application performance is as important as

availability. In another words, they are not willing to compromise performance for

availability. However, performance tradeoffs among different disk technologies are not

always well understood.

Version 2.0 of this whitepaper was written to incorporate four new disk technologies:

High Availability Storage Systems

HP Disk Array with AutoRAID

EMC Symmetrix Integrated Cache Disk Array (ICDA)

HP Fibre Channel-SCSI Disk Multiplexer

Data redundancy is necessary to prevent a single disk failure from causing an outage to

the users. There are two methods available for providing data redundancy: mirrored

standalone disks and Disk Arrays with RAID protection in hardware. Selecting between

these two choices is often more emotional than logical. Each has its place and its pros

and cons. Mirrored standalone disks require the optional product called MirrorDisk/UX.

MirrorDisk/UX can also be used with Disk Arrays to provide double data redundancy.

For example, this might be done to continue data redundancy while the mirrors are split

for an offline backup.