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 1 33
• Data can be copied in both directions, so that if the primary fails and
the replica takes over, data can be copied back to the primary when it
comes back up.
Disadvantages of physical replication in hardware are:
• The logical order of data writes is not maintained in synchronous
replication. When a replication link goes down and transactions
continue at the primary site, writes to the primary disk are queued
in a bit-map. When the link is restored, if there has been more than
one write to the primary disk, there is no way to determine the
original order of transactions. This increases the risk of data
inconsistency.
NOTE Configuring the disk so that it does not allow a subsequent disk write
until the current disk write is copied to the replica (synchronous
writes) can limit this risk as long as the link remains up.
Synchronous writes impact the capacity and performance of the data
replication technology.
Also, because the replicated data is a write to a physical disk block,
database corruption and human errors, such as the accidental
removal of a database table, are replicated at the remote site.
• Redundant disk hardware and cabling are required. This at least
doubles data storage costs. Also, because the technology is in the disk
itself, this solution requires specialized hardware.
• For architectures using dedicated cables, the distance between the
sites is limited by the cable interconnect technology. Different
technologies support different distances and provide different “data
through” performance.
• For architectures using common carriers, the costs can vary
dramatically, and the connection can be less reliable, depending on
the Service Level Agreement.
Advantages of physical replication in software are:
• There is little or no time lag between the initial and replicated disk
I/O, so data remains very current.
• The solution is independent of disk technology, so you can use any
supported disk technology.