Understanding and Designing Serviceguard Disaster Recovery Architectures

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 HP-UX clusters
are either host-based or storage array-based. Extended Distance Cluster always uses host-based
replication (either MirrorDisk/UX or Veritas VxVM mirroring). 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, 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.
• Extended Distance Cluster supports active/active access by implementing SGeRAC, 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 73