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
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
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.
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
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