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

Building an Extended Distance Cluster Using MC/ServiceGuard
Two Data Center Architecture
Chapter 2 57
• All systems are connected to both copies of data, so that if a primary
disk fails but the primary system stays up, there is greater
availabilty because there is no package failover.
The disadvantages of a two-data-center architecture are:
• There is a slight chance of split brain syndrome. Because there are
two cluster lock disks, you would get split brain syndrome if the
following occurred simultaneously:
— All heartbeat networks fail.
— All disk links fail. The disk link from data center A to cluster lock
disk B fails (see Figure 2-1 on page 54.) And the disk link from
data center B to cluster lock disk A fails.
The chances are slight, however these events happening at the same
time would result in split brain syndrome and probable data
inconsistency. Planning different physical routes for both network
and data connections or adequately protecting the physical routes
greatly reduces the possibility of split brain syndrome.
• Software mirroring increases CPU overhead.
• The cluster must be either two or four nodes with cluster lock disks.
Larger clusters are not supported due to cluster lock requirements.
• Although it is a low cost solution, it does require some additional
cost:
— FibreChannel links are required for both local and remote
connectivity.
— All systems must be connected to multiple copies of the data and
to both cluster lock disks.