Understanding and Designing Serviceguard Disaster Recovery Architectures

• 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:
• The time lag between the initial and replicated disk I/O is less, so the replicated data is current
• The solution is independent of disk technology, so you can use any supported disk technology.
• Data copies are peers, so a replica can be re configured to function as a primary disk after
failover.
• There is improvement in read performance, because there are multiple read devices, that is,
the node has access to both copies of data.
• Writes are synchronous unless the link or disk is down.
Disadvantages of physical replication in software are:
• Similar to physical replication in the hardware, the logical order of data writes is not
maintained. 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 cannot be determined
until the resynchronization has completed successfully.
NOTE: Configuring the software so that a write to disk must be replicated on the remote
disk before a subsequent write is allowed can limit the risk of data inconsistency while the
link is up.
• Additional hardware is required for the cluster.
• Distance between sites is limited by the physical disk link capabilities.
• Performance is affected by many factors: CPU overhead for mirroring, double I/Os, degraded
write performance, and CPU time for resynchronization. In addition, CPU failure may cause
a resynchronization even if it is not needed, further affecting system performance.
Logical Data Replication
Logical data replication is a method of replicating data by repeating the sequence of transactions
at the remote site. Logical replication must be performed frequently at both the file system level,
and the database level to replicate all the data associated with an application. Most database
vendors have one or more database replication products. An example is the Oracle Standby
Database.
Logical replication can be configured to use synchronous or asynchronous writes. Transaction
Processing Monitors (TPMs) can also perform logical replication.
12 Disaster Recovery in a Serviceguard Cluster