Designing Disaster Tolerant High Availability Clusters, 10th Edition, March 2003 (B7660-90013)

Disaster Tolerance and Recovery in an MC/ServiceGuard Cluster
Disaster Tolerant Architecture Guidelines
Chapter 142
• Bandwidth affects the rate of data replication, and therefore the
currency of the data should you need to switch control to another
site. The greater the number of transactions you process, the more
bandwidth you will need. The following connection types offer
differing amounts of bandwidth:
— T1 and T3: low end
— ISDN and DSL: medium bandwidth
— ATM: high end
• Reliability affects whether or not data replication happens, and
therefore the consistency of the data should you need to fail over to
the recovery cluster. Redundant leased lines should be used, and
should be from two different common carriers, if possible.
• Cost influences both bandwidth and reliability. Higher bandwidth
and dual leased lines cost more. It is best to address data consistency
issues first by installing redundant lines, then weigh the price of
data currency and select the line speed accordingly.
Disaster Tolerant Cluster Limitations
Disaster tolerant clusters have limitations, some of which can be
mitigated by good planning. Some examples of MPOF that may not be
covered by disaster tolerant configurations:
• Failure of all networks among all data centers — This can be
mitigated by using a different route for all network cables.
• Loss of power in more than one data center — This can be mitigated
by making sure data centers are on different power circuits, and
redundant power supplies are on different circuits. If power outages
are frequent in your area, and down time is expensive, you may want
to invest in a backup generator.
• Loss of all copies of the on-line data — This can be mitigated by
replicating data off-line (frequent backups). It can also be mitigated
by taking snapshots of consistent data and storing it on-line;
Business Copy XP and EMC Symmetrix BCV (Business Consistency
Volumes) provide this functionality and the additional benefit of
quick recovery should anything happen to both copies of on-line data.