Understanding and Designing Serviceguard Disaster Recovery Architectures

Figure 3 Physical Data Replication
Replication
Node 1 Node 2
Replication
Node 1 Node 2
Physical Replication in Software
Physical Replication in Hardware
MirrorDisk/UX is an example of physical replication performed 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 Storage 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:
• Replication consumes no additional CPU.
• The hardware deals with resynchronization if the link or disk fails because 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 disk fails and the replica takes
over, data can be copied back to the primary disk when it comes back up.
Disadvantages of physical replication in hardware are:
• As 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 increases the cost of data storage,
because the technology is in the disk requires specialized hardware.
Disaster Recovery Architecture Guidelines 11