Backing Up Oracle - Computing at Cornell

3.2.1 Disk mirroring
Often, a combination of techniques is used. The techniques we consider are:
Disk mirroring
Offline backup
Online backup
Database export
Full database backup
Partial database backup
Incremental backup
Log file backup
LAN-free backup
Backup using splitcopy features
Backup of RDBMS supporting files
Disk mirroring is a useful technique for maximizing the availability of your
database. Mirroring is the process of writing the same data to multiple
storage devices at the same time. This is done either sequentially, when data
is only written to the mirror once the master write is successful or in parallel
when both master and mirror writes occur at the same time. The first method
is slower but you are more likely to have at least one good copy of the data if
a failure occurs.
When reading from a mirrored logical volume, AIX will read from either the
master or the mirror, whichever is the quickest at the time.
If a media failure occurs, operations are automatically switched to the good
copy and AIX marks the faulty copy as stale. Mirroring allows your users to
continue working even though a media failure has occurred. Mirroring can be
implemented in either software or hardware.
However, mirroring does not remove the need to back up databases. For
example, disk mirroring will not allow you to restore a table that has been lost
or damaged as a result of user error. Also, although disk mirroring
dramatically reduces the impact of media failures, there is still a risk of
damage to both sides of the mirror. If a database is held on one set of
physical volumes, and a mirror image of the same database is maintained on
a separate set of physical volumes, it is possible for both sets of physical
volumes to be damaged or destroyed. This could happen as a result of a
disaster or it could just be bad luck. In such instances, it will be necessary to
recover the database from backup copies.
26 Backing Up Oracle using Tivoli Storage Management