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Chapter 9. Disk Environment Overview
This chapter considers the disk architecture from a DB2 point of view.It focuses
on concepts and recommendationsfor their practical implementation, rather than
on technical details. In order to facilitate the mutual understanding of some
storage terms between DB2 administrators and storage administrators,we
highlight them in italics.Several considerations in this chapter could also apply to
the new tape server environments,such as t he IBM Seascape Virtual Tape
Server.

9.1 Evolution of Disk Architecture

We can identify foursteps in the evolution of the disk architecture that have
progressivelyseparated the concept of volume from the concept of physical
device.

9.1.1 3380 and 3390 Volumes

The 3380 and 3390 havebeen available on the market as physical devices
characterized bya one-to-one relationship between a disk drive and a volume.
The physical characteristicsof these devices also represent a logical view that
consists of:
• Track size (track image), the number of bytes per track: 47476 and 56664
bytes ofdata for 3380 and 3390, respectively
• Capacity in terms of number of tracks or gigabytes
• Device address (device number), which is a thread onto which I/O operations
are serialized by theoperating system
Although the physicaldevices 3380 and 3390 willeventually no longer be used,
the logical view—withthe three characteristics of track size, capacity, and
addressing—continues to existin the new concept oflogical volume or logical
device.

9.1.2 Arrays

An arrayis the combination of two or more physical disk storage devicesin a
single logical device or multiple logical devices.Redundant array of independent
disks (RAID) distributesdata redundantly across an array of disks. The objective
is to achievecontinuous data availability in the face of various hard drivefailures
through the use of disk mirroring, parity data generation and recording, hot
sparing, and dynamic reconstruction of data from a failed disk to a spare disk.
RAID technology provides the disk I/O system with high availability.RAID types
havebeen c ategorizedinto five levels: RAID 1 through 5. Some new definitions
havebeen developed to address new implementations or updated views of the
RAID concept.
Each RAID levelhas some basic characteristics, but all of them have a fixed
mapping between logicaldevices (or logical volumes) and physical drives.
Currently admitteddef initions(see IBM RAMAC Array Subsystem Introduction,
GC26-7004)are:
• RAID 0: data striping without parity