Managing Serviceguard Eighteenth Edition, September 2010

Disk Mirroring
Serviceguard itself does not provide protection for data on your disks, but protection
is provided by HP’s Mirrordisk/UX product for LVM storage, and by the Veritas Volume
Manager for VxVM and CVM.
The logical volumes used for Serviceguard packages should be mirrored; so should
the cluster nodes’ root disks.
When you configure logical volumes using software mirroring, the members of each
mirrored set contain exactly the same data. If one disk fails, the storage manager
automatically keeps the data available by using the mirror. you can use three-way
mirroring in LVM (or additional plexes with VxVM) to allow for online backups or 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. But 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 allows the system to continue normal operation when
a root disk failure occurs.
Disk Arrays using RAID Levels and Multiple Data Paths
An alternate method of achieving protection for your data is to employ a disk array
with hardware RAID levels that provide data redundancy, such as RAID Level 1 or
RAID Level 5. The array provides data redundancy for the disks. This protection needs
to be combined with the use of redundant host bus interfaces (SCSI or Fibre Channel)
between each node and the array.
The use of redundant interfaces protects against single points of failure in the I/O
channel, and RAID 1 or 5 configuration provides redundancy for the storage media.
About Multipathing
Multipathing is automatically configured in HP-UX 11i v3 (this is often called native
multipathing), or in some cases can be configured with third-party software such as
EMC Powerpath.
Redundant Disk Storage 45