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Oracle Tips by Burleson |
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.
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