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  Oracle Tips by Burleson

Oracle disk Raw Devices 

In Unix and NT/Windows systems there are several types of disk formats used. Generally, the highest performance comes from programs that can directly access the disk. In order to be directly accessed a disk must be configured in what as known as raw format meaning no OS buffering or access control is used.

While raw disks provide performance gains over many traditional disk formats they have several limitations that make their use difficult. An example is that only one file may be placed in a raw disk partition at one time, another is that raw disk partitions may require special backup commands. Finally, raw devices can be easily overwritten if the system administrator is not careful.
If you have tuned your application, I/O, and all applicable SGA parameters and still cannot get the performance you want on UNIX or NT, then consider using raw devices. Oracle is capable of reading and writing directly to raw devices. This can increase Oracle performance for disk I/O by over 50 percent and ensures that data integrity is maintained. But when raw devices are used, Oracle datafile names are restricted to a specified syntax. Another limitation is that the entire raw partition has to be used for only one file, which can lead to wasted disk space unless the areas are carefully planned. This will require the Remote DBA to keep an accurate map of which devices belong to which tablespaces, log files, and so on.

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 Remote 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 “Oracle9i 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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