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Donald K. Burleson

Oracle Tips

Using Oracle with Raw Devices

We must always remember that the top cause of slow performance of any Oracle database is disk I/O.  Essentially, any Oracle database is a repository for data, and retrieving data from disk is a very expensive and time-consuming operation.

I like it RAW

Because of the high amount of I/O that many Oracle systems experience, many Oracle professionals may consider the use of “raw” devices in conjunction with large block sizes (db_block_size). A raw device is defined as a disk that bypasses the I/O overhead created by the Journal File System (JFS) in UNIX.

The reduction in disk I/O overhead can improve throughput, but only in cases where I/O is already the bottleneck for the Oracle database. Furthermore, raw devices require a tremendous amount of manual work for both the Oracle administrator and the systems administrator.

It is critical to know that Oracle recommends that raw devices should only be considered when the Oracle database is I/O bound. However, for these types of Oracle databases, raw devices can dramatically improve overall performance. If the database is not I/O bound, switching to raw devices will have no impact on performance.

It is worth mentioning here the feature in Oracle 8.1.7 called directIO supported on some of the top UNIX platforms. Veritas also sells a product called QuickIO. These options give Oracle near RAW performance without the additional overhead in managing RAW devices.

In summary, raw devices for Oracle databases can provide improved I/O throughput only for databases that are already I/O bound.

However, this performance gain from raw devices comes at the expense of increased administrative overhead for the Oracle administrator. We also know that raw devices will only improve the performance of Oracle databases whose Oracle subsystem is clearly I/O bound.

In sum, for systems that are not I/O bound, moving to raw devices will not result in any performance gains.

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