Databases Reference
In-Depth Information
Oracle recommends that you size the fast recovery area large enough that it equals the sum of the size of the
database plus the size of the archived redo logs not yet copied to tape and the size of any incremental backups.
Although the fast recovery area is by no means mandatory, Oracle recommends that you use one. You must have
activated a fast recovery area in order to avail of the flashback database or the guaranteed restore point feature. In
addition, using a fast recovery area means you're reducing your recovery time, since necessary backups and archived
redo logs can be kept on disk instead of having to recover from tape backups. Since obsolete backups are automatically
deleted when space is needed for fresh files, you won't be running the risk of accidentally deleting necessary files.
Make and Protect a Database Redundancy Set
You may have to perform a database recovery when you lose or can't access (because of a media problem) any of these
three types of Oracle database files: data files, online redo log files, and control files. Oracle recommends that you
maintain a
database redundancy set
, which is a set of files that'll help you recover any of the three key types of Oracle
files when they become unavailable to the database. This essential set of recovery-related files, called the
redundancy
set, will enable you to recover your database from any contingency. Here are the components of the redundancy set:
•
Most recent backups of all data files plus the control file
•
All archived redo logs made after the last backup
•
Current control files and online redo file copies
•
Oracle database-related configuration file copies (spfile, password file, tnsnames.ora, and
listener.ora files, for example)
To maintain the database redundancy set described here, you must duplex the control file as well as the online
redo log files
at the database level
. That is, although a mirrored disk setup means that a copy of the redo log files and the
control file will be automatically made at the operating-system level, that doesn't provide you with complete safety.
Although you can mirror the online redo files at the operating-system level, Oracle advises against this. Follow
these Oracle best practices for protecting your database files:
•
Multiplex the online redo log file at the database level. If you're using the fast recovery area,
make this the destination for the duplexed copies of the online redo log file.
•
Ensure that you use hardware or software (OS) mirroring to duplex the control file. This way
the database will always continue to operate following the loss of one control file.
•
Mirror the data files in the database so you don't have to perform media recovery for simple
disk failures.
•
Keep more than one set of backups so you can withstand a database corruption issue.
•
Consider making more than one copy of the redundancy set on tape if you aren't going to be
using a disk-based recovery plan.
Oracle recommends that you use at least two disk drives on all production systems (one for the redundancy
set and the other for the data files) and completely separate them by using different volumes, file systems, disk
controllers, and RAID devices to hold the two sets of files: database files and the files in the redundancy set. One way
to do this is to simply use the Oracle recommended fast recovery area. In fact, Oracle recommends the fast recovery
area as a logical candidate to keep a copy of all the files belonging to the redundancy set (which includes the most
recent database backup) on disk.
