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 140
Figure 1-7 Physical Data Replication
MD Software RAID is an example of physical replication done in the
software; a disk I/O is written to each array connected to the node,
requiring the node to make multiple disk I/Os. Continuous Access XP on
the HP StorageWorks Disk Array XP series is an example of physical
replication in hardware; a single disk I/O is replicated across the
Continuous Access link to a second XP disk array.
Advantages of physical replication in hardware are:
• There is little or no lag time writing to the replica. This means that
the data remains very current.
• Replication consumes no additional CPU.
• The hardware deals with resynchronization if the link or disk fails.
And resynchronization is independent of CPU failure; if the CPU
fails and the disk remains up, the disk knows it does not have to be
resynchronized.
• 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: