Understanding and Designing Serviceguard Disaster Recovery Architectures

NOTE: For the most up-to-date support and compatibility information see the SGeRAC for SLVM,
CVM and CFS Matrix and Serviceguard Compatibility and Feature Matrix on http://www.hp.com/
go/hpux-serviceguard-docs -> HP Serviceguard Extension for RAC.
1– The Cross Subnet feature is enabled beginning with Serviceguard A.11.18, and with appropriate
patches on HP-UX . For more information on the Cross Subnet feature on Linux, see HP Serviceguard
A.11.20.10 for Linux Release Notes available at www.hp.com/go/linux-serviceguard-docs.
Differences Between Extended Distance Cluster and Metrocluster
The major differences between an Extended Distance Cluster and a Metrocluster are:
• The methods used to replicate data between the storage devices in the two data centers. The
two basic methods available for replicating data between the data centers for Serviceguard
clusters are either host-based, or storage array-based. Extended Distance Cluster always uses
host-based replication (either MirrorDisk/UX or Veritas VxVM mirroring on HP-UX, and Disk
(MD software RAID) for Linux). Any (mix of) Serviceguard supported storage can be
implemented in an Extended Distance Cluster. Metrocluster always uses array-based replication
or mirroring, and requires storage from the same vendor in both data centers (that is, a pair
of XPs with Continuous Access, a pair of Symmetrix DMX arrays with SRDF, or a pair of EVAs
with Continuous Access).
• Data centers in an Extended Distance Cluster can span up to 100km, whereas the distance
between data centers in a Metrocluster is defined by the shortest (up to 300km) of the following
distances:
◦ the maximum distance that guarantees a round trip network latency of no more than
200ms
◦ the maximum distance supported by the data replication link
◦ the maximum supported distance for WDM as stated by the provider
• In an Extended Distance Cluster on HP-UX, there is no built-in mechanism for determining the
state of the data being replicated. When an application fails over from one data center to
another, the package can start up if the volume groups can be activated. A Metrocluster
implementation provides a higher degree of data integrity; that is, the application is only
allowed to start up based on the state of the data and the disk arrays.
In an Extended Distance Cluster on Linux, you can control the threshold of data loss using
RPO_target.
• Extended Distance Cluster supports active/active access, whereas Metrocluster supports
active/standby access.
• Extended Distance Cluster disk reads may outperform Metrocluster in normal operations. On
the other hand, Metrocluster data resynchronization and recovery performance are better
than Extended Distance Cluster.
Differences Between Extended Distance Cluster and Metrocluster 75