RAC Overview:
RAC means that two or more computers are sharing a common set of
disk. All of the computers have read
write access to the data in the single (one) database. If one computer (node)
fails in the cluster - work continues as before - the database never goes down
Good: RAC is a good solution for hardware failures, system failures, and
software failures.
Good: Sharing resources across servers.
Good: RAC addresses system failures such as s node failures, or instance
crashes.
Good: load balancing: you have multiple instances running on the same
database and any new connection is routed to the instance with less work load
in the available instances.
Disadvantage: RAC is very expensive – you need many servers, fiber channel
infrastructure, eventually Storage Area Network. Of course this cost money.
Data Guard Overview:
Data guard is a configuration, which has at least one standby database
of the primary database. The primary database can have one or more standby
databases.
Good: This enables recovery from site
disasters (fire, flood, explosion, whatever) oracle data corruptions.
Good: Can use the data guard site for
read-only reporting purposes if we required.
Good: Data Guard provides data
protection.
Disadvantage: The Data Guard option is only
available in the enterprise edition of oracle software.
Few Difference between Oracle RAC and Data
Guard:
- RAC has one database and several instances associates with it, but data guard has several databases (one primary and others standby databases).
- RAC is the recommended solution for instance, software and hardware level failures. Data guard is the recommended solution for the SITE failures.
- RAC must have a shared storage, which can be accessed from all the nodes of the system, but in data guard there is no shared storage, which is common for all the sites.
- RAC is Active-Active solution, expensive cost wise:: Data guard is Active-Passive , less expensive compare than RAC.
- RAC nodes are placed within the same premises (physically close together): Data Guard site can be physically remote (different sites at unlimited distances from each other).
- RAC licensing is more expensive than Data Guard licensing: Data Guard is not an extra cost option, but you need to pay for a full oracle license at the standby node.
