Understanding and Designing Serviceguard Disaster Tolerant Architectures Fourth Edition, December 2007 (T1906-90023)

Disaster Tolerance and Recovery in a Serviceguard Cluster
Understanding Types of Disaster Tolerant Clusters
Chapter 128
be implemented in an Extended Distance Cluster. Metrocluster
always uses array-based replication/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 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 of the following distances:
— the maximum distance that guarantees a network latency of no
more than 200ms
— the maximum distance supported by the data replication link
— the maximum supported distance for DWDM 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 is
allowed to start up if the volume group(s) 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.
Continental Cluster
A continental cluster provides an alternative disaster tolerant
solution in which distinct clusters can be separated by large distances,
with wide area networking used between them. Continental cluster
architecture is implemented via the Continentalclusters product,
described fully in Chapter 2 of the Designing Disaster Tolerant HA
Clusters Using Metrocluster and Continentalclusters user’s guide. The
design is implemented with distinct Serviceguard clusters that can be
located in different geographic areas with the same or different subnet