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Donald K. Burleson

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Oracle and Disk Usage

As you can see, an attempt is made to spread I/O between the two areas. Indexes and data are on separate controllers, as are redo logs and rollback segments. Additional recoverability in case of disk crash is given by having exports on one drive and archive log files on the other. While infinitely better than only one area, having only two areas is still an extremely vulnerable condition and is not recommended. Again, with the most up-to-date storage areas with large caches, extreme striping, and hot replacement of disks, one or two areas is workable.

Three Areas (Nearly There) 

With three areas available we improve the chances that the database can be recovered from disk crashes. We can also reduce the disk contention caused by sharing areas between highly active files in flagrant disregard for OFA rules. Let’s look at the three-area layout:

AREA1. Executables, redo logs, rollback segments, export files, copy of the control file
AREA2. Data-datafiles, temporary user datafiles, a copy of the control file, redo logs
AREA3. Archive log files, indexes, a copy of the control file

Again, an attempt is made to spread I/O evenly across the platters. While this is better than one or two areas, there is still contention between redo logs and rollback segments, indexes, and archives.

Four Areas (Just about Right) 

Four areas are much better. Now we can spread the read- and write-intensive rollbacks and redo logs. In addition, we can isolate the archives away from the indexes. Let’s look at the structure:
 

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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