SCF Reference Manual for the Storage Subsystem (G06.24+, H06.03+)

Introduction to the Storage Subsystem
SCF Reference Manual for the Storage Subsystem—529937-007
1-6
Fault Tolerance
Fault Tolerance
An important concept, and a possible source of confusion for users, is the three ways
that the word primary can be used when configuring and managing devices.
Primary Processor
An IOP runs in two processors: primary and backup. The PRIMARYCPU and
BACKUPCPU attributes of the ADD command are set when the device is configured.
To change which processor runs the primary process, specify that processor number in
a PRIMARY command. For example, see Swapping Processors for a Disk on
page 7-24. However, the PRIMARY command does not change the configured value of
the PRIMARYCPU or BACKUPCPU attributes.
Primary Path
A nonmirrored device can be accessed through two paths: a primary path and a
backup path. To set the paths use these attributes of the ADD command when the disk
is configured:
PRIMARYLOCATION PRIMARYSAC
BACKUPLOCATION BACKUPSAC
The primary and backup paths go to the primary device.
A mirrored device, such as a mirrored disk, can be accessed through an additional two
paths: a mirror path and a mirror-backup path. The paths are set by these attributes of
the ADD command when the device is configured:
MIRRORLOCATION MIRRORSAC
MBACKUPLOCATION MBACKUPSAC
The primary and backup paths go to the primary device. The mirror and mirror-backup
paths go the mirror device.
You can make two paths to a mirror volume active at the same time: one path to the
primary device (either -P or -B) and one path to the mirror device (either -M or -MB). To
make a path active, use the SWITCH command. For an example, see Changing the
Active Path for a Magnetic Disk on page 7-27.
Primary Disk
A mirrored disk volume has two disks: a primary disk and a mirror disk. This logical
combination of two magnetic disks is set by the PRIMARYLOCATION and
MIRRORLOCATION attributes of the ADD command when the disk is configured.