VERITAS Volume Manager 3.1 Reference Guide

Table Of Contents

116 Chapter3

Disk Arrays

Multipathed Disk Arrays

Some disk arrays provide multiple ports to access their disk devices.

These ports, coupled with the HBA (Host Bus Adaptor) controller and

any array-local data bus and I/O processor, make up multiple hardware

paths to access the disk devices. Such disk arrays are called

multipathed disk arrays

. This type of disk array can be connected to

host systems in many different conﬁgurations, (such as multiple ports

connected to different controllers on a single host, chaining of the ports

through a single controller ona host, or ports connected to differenthosts

simultaneously).

Multipathed disk arrays can be classiﬁed as either Active/Active type, or

Active/Passive type.

Active/Passive Type Disk Arrays

This type of disk array designates one of the multiple paths to a disk

device as the primary path, and the others as secondary paths. Access to

a disk is enabled through its primary path. If the primary path fails, one

of the secondary paths is made the new primary path to the disk device,

either automatically, or through administrator intervention. Access to

the disk device through a secondary path may be disabled, or may

degrade system performance severely.

In this type of disk array, a disk device is bound to one of the redundant

hardware components local to the disk array,( for example, the I/O bus,

I/O controller, cache and access port). This disk is then said to be owned

by the port, and the I/O path through that port is designated as the

active path

Some disk arrays have a mode setting, called autotresspass mode, in

which the disk arrays automatically mark a path as primary path when

I/O is attempted through that path. This switch of primary path is an

expensive operation, and degrades the performance of the disk array

severely if different paths are used to access the disk alternately, by the

same or different hosts.