Databases Reference
In-Depth Information
We use the Oracle Database 12
c
release throughout this topic, thus providing you with cutting-edge RMAN
backup and recovery solutions. Most of what we say, however, applies equally to Oracle Database 11
g
. We specifically
mention whenever we're discussing a feature not available in Oracle Database 12
c.
Types of Database Failures
Since database backups are made to protect against a database failure, let's quickly review the types of database
failures that can occur. A database can fail, either entirely or partially, for various reasons. You can recover from some
types of database failure with scarcely any effort on your part, because the Oracle database can recover automatically
from some types of failures. The more critical types of failures require you to go in and “recover” the database by using
your backups. You can divide database failures into the categories covered in the following sections.
Statement Failure
A typical example of a statement failure is when a program attempts to enter invalid data into an Oracle table. The
statement will fail because of the checks built into the data insertion process. The solution here is to clean up the data
by validating or correcting it. Sometimes a program may fail to complete because of programmatic logical errors. You
must then refer the problem to the development group for corrections.
It is fairly common for a long data insertion job or a data import job to fail midway because there is no more room
to put in the data. If you haven't already invoked the
resumable space allocation
feature, you must add space to the
relevant tablespace. Another common cause of a statement failure is not having the proper privileges to perform a task.
Your task as a DBA is to simply grant the appropriate privileges for the user who invoked the failed SQL statement.
User Process Failure
Sometimes a user process may be terminated abruptly because of, say, the user performing an abnormal disconnect
or performing a terminal program error and losing the session connection. As a DBA, there is not much you need to do
here: the Oracle background processes will roll back any uncommitted changes to the data and release the locks that
were held by the abnormally disconnected user session. The user will have to reconnect after the abrupt termination.
Network Failure
A network failure can also cause a database failure. Network failures can occur because the Oracle Net listener, the
network interface card (NIC), or the network connection has failed. The DBA must configure multiple network cards
and a backup network connection and backup listener to protect against these errors. In addition, you can use the
connect-time failover feature to protect against a network failure.
Instance Failure
You experience an Oracle instance failure when your database instance comes down because of an event such as
a hardware failure, a power failure, or an emergency shutdown procedure. You may also experience an instance
shutdown when the key Oracle background process, such as
PMON
, shuts down because of an error condition.
Following an instance failure, first you check the alert log and trace files for any potential hints about the cause of
the instance failure. Following this, you can just restart the database instance by using the Oracle command
startup
from the SQL*Plus command line.
Since the database wasn't cleanly shut down and the database files aren't synchronized, Oracle will perform an
automatic instance or crash recovery at this point. Oracle will automatically perform a rollback of the uncommitted
transactions by using data from the undo segments and will roll forward the committed changes it finds in the online
