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Donald K. Burleson

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Oracle disk Raw Devices

 Another method is to turn off UNIX buffering. Whether the option of removing UNIX buffering is open to you depends on the version of UNIX you are using.

There are also limitations on types of backup that can be used. Many third-party software packages that are designed for use with Oracle support backup of RAW devices. If you don’t have one of these packages, I suggest ensuring you have enough formatted (cooked) file systems to support a “dd” to a cooked file system followed by a normal backup.

There is some debate as to whether the reported up-to-50 percent increase in speed of access is due to the RAW device usage, or a good deal of it is an artifact of the conversion process from a cooked to araw system. Generally, a system with bad performance has other problems, such as chained rows and excessive table extents as well as improper placement of indexes, tables, redo, and rollback. The DBA converts to raw by exporting, dropping the database, doing the raw partitions, re-creating the database, and then importing. Usually, files will be better placed due to lessons learned. The chained rows and multiple table extents are eliminated by the export/import; and another major performance problem, brown indexes (the process by which excessive numbers of empty leaf nodes resulting from UPDATE and DELETE operations cause index broadening), is fixed by the import rebuild of the indexes. Voila! The system is 50 percent faster, and RAW gets the credit, when doing all of the above to the database on a cooked file system would have given the same improvements.

If you want to use a shared instance (Oracle’s Parallel Server or Real Application Clusters option), you must use raw devices on UNIX since there are no UNIX file systems that support the proper sharing of disks in other than a raw state.

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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