Drawbacks of RAW Devices

The disadvantages of using raw devices are quite prominent to prominent enough to discourage most sites from going raw.

Greater maintenance difficulty:

Most of the datafile management flexibility offered by Oracle can not be expoited with raw devices. Any site whose performance has been affected by an I/O bottleneck appreciates the importance of the springiness in moving datafiles around. If a disk is found to be busy in excess of 90 percent most of the time such disks are denoted to as hot disks, certain datafiles on that disk can be transferred to another disk that is not so busy.

With a regular file system, this task is comparatively easy, provided that adequate space is presented on the target disk. However, raw devices introduce new considerations and complexities. Since each file is stored in a separate raw partition, a free raw partition that can accommodate the data file has to be available on the target disk. For instance, if the datafile on the hot disk is 10GB, and the raw partition on which it is is present is 10GB plus 1MB the additional 1MB is for raw partition overhead, a raw partition of the same size of bigger needs to be available on the target disk. This sounds easy enough; but considering the disparity in the size of various Oracle tablespaces, maintaining free raw partitions catering to the datafiles of each tablespace can be a daunting task, especially for larger databases. Capacity planning becomes extremely tedious, since raw partitions of the same size of larger have to be made available for database expansion and disk I/O balancing without wasting disk space.

Further, many standard, easy to use and remember commands, such as cp, mv, rm, and so on, do not work with raw devices, introducing the need for moderately complex commands and tools to administer the Oracle database.

Absence of the File system buffers:

Ironically, the feature that gives raw devices their claim to fame can also hamper performance. In applications both OLTP and DSS in which data that is read or written may be required again shortly, raw devices require another disk read to obtain the data if the data is not available in System Global Area SGA. File system buffers may help in such scenarios, since data is held in the buffers for a little while, until the buffer space is reused for something else data. If the required data is already in the file system buffers, and additional disk read or write can be avoided because the operating system will retrieve the data from the buffers.

As you can see, before deciding to use raw devices you must prudently evaluate your environment and necessities. Other that RAC environment, in which raw devices are mandatory, applications rarely call for the rigidity of raw devices. And Oracle supports direct disk I/O. The availability of direct I/O features in the operating system makes raw devices almost redundant. As was mentioned earlier, however, if the transaction rates of an application are so high that they are expected to cause of are causing Input/output bottlenecks, raw device usage might be worthwhile. This can be established by pre-production benchmarking. Using automated tools, you can pretend production level transaction loads and determines whether I/O bottlenecks are occurring in spite of the best physical data file settlement opportunity. You should also take into account that high transaction volumes may be caused by the SQL statements/queries in use. Tuning the SQL statements may reduce transaction loads.

In Short:

If you are a real raw supporter and remain convinced that you need to use raw devices, or if you are already past the point of easy return in using them, you might find the some helpful tips from our other articles or from www.dbametrix.com particularly while managing critical production environment which are running round the clock.

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