Disaster Tolerance and Recovery in a Serviceguard Cluster

Disaster Tolerant Architecture Guidelines

•Because there are multiple read devices, that is, the node has access to both copies of data, there may be improvements in read performance.

•Writes are synchronous unless the link or disk is down.

Disadvantages of physical replication in software are:

•As with 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 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 often must be done at both the file system level, and the database level in order to replicate all of 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.

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Chapter 1