Understanding and Designing Serviceguard Disaster Recovery Architectures

Benefits of Metrocluster
The following are the benefits of Metrocluster:
• Metrocluster offers a more resilient solution than Extended Distance Cluster, because provides
full integration between Serviceguard’s application package and the data replication subsystem.
The storage subsystem is queried to determine the state of the data on the arrays. In a
Metrocluster configuration, application data is replicated between two data centers.
Metrocluster evaluates the status of the local and remote copies of the data, including whether
the local site holds the primary copy or the secondary copy of data, whether the local data
is consistent or not and whether the local data is current or old. Based on the result of this
evaluation, Metrocluster checks if it is safe to start the application package, whether a
resynchronization of data is needed before the package can start, or whether manual
intervention is required to determine the state of the data before the application package is
started. Metrocluster allows for customization of the startup behavior for application packages
depending on your requirements, such as data currency or application availability. This means
that by default, Metrocluster will always prioritize data consistency and data currency over
application availability. If, however, you choose to prioritize availability over currency, you
can configure Metrocluster to start up even when the state of the data cannot be determined
to be fully current (but the data is consistent).
• Metrocluster can be configured using any one of the replication modes supported by Continuous
Access XP or P9000, Continuous Access EVA or P6000, or EMC SRDF. Currently, Metrocluster
with 3PAR Remote Copy supports only synchronous replication mode. These replication modes
enable you to prioritize performance and data currency between data centers based on your
needs. Details on the replication modes supported are available in the Compatibility Feature
Matrix documents. For the Compatiblility Feature Matrix, see www.hp.com/go/
hpux-serviceguard-docs and select the respective Metrocluster Product.
• Metrocluster may provide significantly better performance than Extended Distance Cluster
during recovery because data replication and resynchronization are performed by the storage
subsystem. Unlike Extended Distance Cluster, Metrocluster does not require any additional
CPU time, which minimizes the impact on the host.
• The data written to the local copy is updated to the remote copy with little or no lag time. So,
the data remains current in the remote copy as well. Data can be copied in both directions,
so that if the primary site fails and the replica takes over, data can be copied back to the
primary site when it comes back up. Disk resynchronization is independent of CPU failure
(that is, if the hosts at the primary site fail but the disk remains up, the disk does not need to
be resynchronized).
• Metrocluster with Continuous Access for P9000 and XP is supported in a Three Data Center
solution, providing the data consistency of synchronous replication and the capability of
Continuous Access journaling replication to protect against local and wide-area disasters. The
Three Data Center solution integrates Serviceguard, Metrocluster with Continuous Access for
P9000 and XP, Continentalclusters and HP Storage XP 3DC Data Replication Architecture.
This configuration consists of two Serviceguard clusters. The first cluster, which is basically a
Metrocluster, has two data centers namely Primary data center (DC1) and Secondary data
center (DC2). The second cluster, a normal Serviceguard cluster, has only one data center
namely Third data center (DC3). Continuous Access synchronous replication is used within
the Metrocluster region and Continuous Access long-distance journal replication is used
between the Metrocluster and recovery cluster regions.
• Metrocluster supports Data Replication Storage Failover Preview; this allows you to preview
the preparation for the storage of the data replication environment in a Metrocluster failover
or Continentalclusters recovery. See “Data Replication Storage Failover Preview” (page 35).
• Metrocluster supports for Synchronous Optical Network and Synchronous Digital Hierarchy
(SONET/SDH) interconnect.
26 Metrocluster and Continentalclusters