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Oracle Tips by Burleson Consulting
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The JFS buffer and Oracle
raw devices
Because of the high amount of I/O that many
Oracle systems experience, many Oracle 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.
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