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Donald K. Burleson
Oracle Tips |
Oracle Redo Log files
As their name implies, redo
logs are used to restore transactions after a system crash or other
system failure. The redo logs store data about transactions that
alter database information. According to Oracle, each database
should have at least two groups of two logs each on separate
physical non-RAID5 drives, if no archive logging is taking place,
and three or more groups when archive logging is in effect. These
are relatively active files, and, if made unavailable, the database
cannot function. They can be placed anywhere except in the same
location as the archive logs. Archive logs are archive copies of
filled redo logs and are used for point-in-time recovery from a
major disk or system failure. Since they are backups of the redo
logs, it would not be logical to place the redo logs and archives in
the same physical location. The size of the redo logs will determine
how much data is lost as a result of a disaster affecting the
database. I have found three sets of multiplexed logs to be the
absolute minimum to prevent checkpoint problems and other
redo-related wait conditions; under archive log, the use of three
groups is a requirement.
Oracle Control Files
An Oracle database cannot be started without at least one control
file. The control file contains data on system structures, log
status, transaction numbers, and other important information about
the database. When tools such as Oracle’s RMAN backup tool are used,
the control file can be several tens of megabytes in size. It is
wise to have at least two copies of your control file on different
disks, three for OFA compliance. Oracle will maintain them as mirror
images of each other. This ensures that loss of a single control
file will not knock your database out of the water. You should not
bring a control file back from a backup, even though this is
supported; it is a living file that corresponds to current database
status. In both Oracle8i and Oracle, there is a CREATE CONTROL
FILE command that allows recovery from loss of a control file.
However, you must have detailed knowledge of your database to use it
properly. There is also an ALTER DATABASE BACKUP CONTROL FILE TO
TRACE command that creates a script to rebuild your control file. In
Chapter 15, Backup and Recovery Procedures for Oracle, the section
titled “Backup and Recovery Plans for Oracle,” explains in detail
how to protect yourself from the loss of a control file. It is
easier to maintain extra control file copies.
This is an
excerpt by Mike Ault’s book “Oracle
Administration & Management”. If you want more current Oracle tips
by Mike Ault, check out his new book “Mike
Ault’s Oracle Internals Monitoring & Tuning Scripts” or
Ault’s Oracle Scripts Download.
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