NonStop Systems Introduction

NonStop Server Architecture

NonStop Systems Introduction—527825-001

7-8

Mirrored Disk Volumes

Disk drives represent the most critical class of I/O devices in a NonStop server,

because applications are constantly accessing and updating the databases that reside

on disk drives.

Although the architecture provides dual paths to any disk, what happens if the disk

drive itself fails? This scenario could prove very serious, because some applications

must have guaranteed access to the database at all times. For example, if a hospital’s

patient-monitoring system cannot access the database containing the patient’s history,

the patient’s life could be in danger. For situations like these, you can provide the

highest possible degree of fault tolerance by mirroring the disk drives that hold the

data.

A mirrored disk volume is two separate disk drives that are treated as a single drive by

the system. With mirrored disk volumes, all data written out to the user’s files by

applications is automatically written onto both disk drives. When applications need to

read data from the files, they can access whichever drive will provide quickest access.

Such access is possible because both drives have identical data.

Each disk drive in the mirrored volume is attached to two ServerNet addressable

controllers. If the primary disk drive fails, applications can continue to read and write

data using the backup disk drive.

Figure 7-6. Wormhole Routing

Any input to any output Full duplex

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