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The JFS buffer and Oracle raw devices
Oracle Tips by Burleson Consulting


The JFS buffer and Oracle raw devices

Because of the high amount of I/O that many Oracle systems experience, many Oracle Remote DBAs consider the use of “raw” devices. 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 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. 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.

In many UNIX environments such as AIX, raw devices are called virtual storage devices (VSDs). These VSDs are created from disk physical partitions (PPs), such that a single VSD can contain pieces from several physical disks. It is the job of the system administrator to create a pool of VSDs for the Oracle administrator. The Oracle administrator can then take these VSDs and combine them into Oracle datafiles. This creates a situation where an Oracle datafile may be made from several VSDs. This many-to-many relationship between Oracle datafiles and VSDs makes Oracle administration more challenging.

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

Now that we have a general idea of how UNIX tasks operate, let’s take a look at how RAM memory is managed in UNIX.

The above is an excerpt from the "Oracle9i UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.


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