HP Serviceguard Extended Distance Cluster for Linux A.01.00 Deployment Guide, Second Edition, May 2008

Disaster Tolerance and Recovery in a Serviceguard Cluster
Disaster Tolerant Architecture Guidelines
Chapter 1 41
• The logical order of data writes is not always 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, then there is no way to
determine the original order of transactions until the
resynchronization has completed successfully. This increases the risk
of data inconsistency.
Also, because the replicated data is a write operation 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.
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.
• Redundant disk hardware and cabling are required. This, at a
minimum, doubles data storage costs, because the technology is in
the disk itself and 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.
• Data copies are peers, so there is no issue with reconfiguring a
replica to function as a primary disk after failover.