Managing Serviceguard 11th Edition, Version A.11.16, Second Printing June 2004

Understanding Serviceguard Hardware Configurations
Redundant Disk Storage
Chapter 2 45
shared bus. All SCSI addresses, including the addresses of all interface
cards, must be unique for all devices on a shared bus. See the manual
Configuring HP-UX for Peripherals for information on SCSI bus
addressing and priority.
Data Protection
It is required that you provide data protection for your highly available
system, using one of two methods:
• Disk Mirroring
• Disk Arrays using RAID Levels and Multiple Data Paths
Disk Mirroring
Disk mirroring is one method for providing data protection. The logical
volumes used for Serviceguard packages should be mirrored.
Serviceguard does not provide protection for data on your disks. This
capability is provided for LVM storage with HP’s MirrorDisk/UX product,
and for VxVM and CVM with the VERITAS Volume Manager
(B9116AA). When you configure logical volumes using software
mirroring, the members of each mirrored set contain exactly the same
data. If one disk should fail, the storage manager will automatically keep
the data available by accessing the other mirror. Three-way mirroring in
LVM (or additional plexes with VxVM) may be used to allow for online
backups or even to provide an additional level of high availability.
To protect against Fibre Channel or SCSI bus failures, each copy of the
data must be accessed by a separate bus; that is, you cannot have all
copies of the data on disk drives connected to the same bus.
It is critical for high availability that you mirror both data and root
disks. If you do not mirror your data disks and there is a disk failure, you
will not be able to run your applications on any node in the cluster until
the disk has been replaced and the data reloaded. If the root disk fails,
you will be able to run your applications on other nodes in the cluster,
since the data is shared. However, system behavior at the time of a root
disk failure is unpredictable, and it is possible for an application to hang
while the system is still running, preventing it from being started on
another node until the failing node is halted. Mirroring the root disk can
allow the system to continue normal operation when a root disk failure
occurs, and help avoid this downtime.