The “recovery” portion of backup and recovery tasks includes two major
types of activities: restoring and recovering. Restoring a file is the
process of copying a backup into place to be used by the database. This is
necessary if, for example, a file is damaged because the physical disk it is on
fails. This is usually due to hardware problems, such as disk write errors, or
controller failure. In that case, a backup of the file needs to be copied onto
a new (or repaired) disk.
Recovering the file entails applying redo such that the state
of the file is brought forward in time, to whatever point you want. That point
is usually as close to the time of failure as possible.
In the database industry, these two operations are often referred to,
collectively, with the single term “recovery.”
Causes
of File Loss
Files can be lost or damaged due to:
•
User
error: An administrator may inadvertently delete or copy over a necessary
operating system file.
•
Application
error: An application or script can also have a logic error in it, as it
processes database files, resulting in a lost or damaged file.
•
Media
failure: A disk drive or controller may fail fully or partially, and introduce
corruption into files, or even cause a total loss of files.
Critical
Versus Noncritical
A noncritical file is one that the database and most applications can
operate without. For example, if the database loses one multiplexed redo log
file, there are still other redo log file copies that can be used to keep the
database operating.
Although the loss of a noncritical file does not cause the database to
crash, it can impair the functioning of the database. For example:
•
The loss of an index tablespace can cause
applications and queries to run much slower, or even make the application
unusable, if the indexes were used to enforce constraints.
•
The loss of an online redo log group, as long as it
is not the current online log group, can cause database operations to be
suspended until new log files are generated.
•
The loss of a temporary tablespace can prevent users
from running queries or creating indexes until they have been assigned to a new
temporary tablespace.
You fix the problem by taking one of these actions:
•
Create a new file.
•
Rebuild the file.
•
Recover the lost or damaged file.
