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Donald K. Burleson

Oracle RAC Tips

Database Availability

We need to realize that database servers or systems are required to be online with no downtime. There are two types of downtime: planned and unplanned. Both types of downtime have similar effects, in that they remove the database from service. However, planned downtime minimizes the damage by planning and scheduling the outage. Both planned and unplanned downtimes have significant impact on the smooth running of business applications.

When we discuss the availability of databases, we need to look at the total environment and infrastructure in which a typical database is located. Typically a database is an application hosted on a server. The database application has its own availability features that are unique from the system availability point of view. There are three situations that we need to bear in mind.

  • Database availability is a function of system, or more appropriately server, availability. If the server is inoperable for any reason, the database is not available or usable.
  • The server is up and running and the database is functioning as expected, but end users or applications are unable to access the database because of failures in the networking path. In this case, even though the system and/or database are highly available, the data service is not reachable.
  • The database is a logical structure for the physical data store that resides on the physical disks (also known as the storage device). If for some reason, the storage unit fails, or the server is unable to reach or connect to the storage unit, the database cannot be brought up. As a result database service is not available for use.

The above factors or situations highlight the necessary planning required in terms of server availability, storage availability, and network availability. Chapter 13, RAC Design Considerations, discusses more details.


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