||Oracle Tips by Burleson
Oracle10g Grid Computing
Chapter 3 - High
Performance and Highly Available
Oracle Database Service in HA Cluster
Multiple Instances exist at the same time
accessing a single database. (Data files are common to the
Multiple nodes have read/write access to
the shared storage at the same time. Data blocks are read and
updated by multiple nodes.
Should a failure occur in a node and the
oracle instance is not usable or has crashed, the surviving node
performs recovery for the crashed instance. There is no need to
restart the instance on the surviving node, a parallel instance
is already running there.
All the client connections continue to
access the database through the surviving node/instance. With
the help TAF (Transparent Application Failover) facility,
clients will be able to move over to the surviving instance near
There is no such thing as the moving of
Volumes and File system to the surviving node.
Issues with FO Clusters – Hidden
No doubt, there is wide acceptance of failover
clusters, and in most situations, FO clusters provide an excellent
failover mechanism, but there are some hidden risks with the nature
and implementation of FO clusters.
The hidden risks with FO clusters are as
Database Failover is a time consuming
process. It is usually a cold a fail-over (Active/passive). A
fresh database instance starts on the surviving node. For
example, the un-mounting of a file system may hang and as a
result, off-line activity may take longer time.
The entire process of monitoring and
failover depends on scripts. Scripts can become problematic with
bugs and sometimes they may not take care of all situations.
Cluster technology cannot protect against
software corruption and human-induced failures. If the server
operating system crashes in such a manner that it corrupts the
file system, recovery by the other member of a cluster may not
If the owner of a file accidentally deletes
it from the file system, the cluster will be unable to recover
the file. Even after failing over to second node, the same
problem of file loss exists.
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