Donald K. Burleson
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
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
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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