HP Serviceguard manual Data Protection, Disk Mirroring, About Multipathing

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

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.

NOTE: 4.1 and later versions of Veritas Volume Manager (VxVM) and Dynamic Multipathing (DMP) from Symantec are supported on HP-UX 11i v3, but do not provide multipathing and load balancing; DMP acts as a pass-through driver, allowing multipathing and load balancing to be controlled by the HP-UX I/O subsystem instead.

For more information about multipathing in HP-UX 11i v3, see the white paper HP-UX 11i v3 Native Multi-Pathing for Mass Storage, and the Logical Volume Management volume of the HP-UX System Administrator’s Guide at the address given in the preface to this manual. See also “About Device File Names (Device Special Files)” (page 80).

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