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Raw Devices

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Raw Devices have been in use for a very log time. They were the primary storage structures for data files of the Oracle Parallel Server. They remain in use even in the RAC versions 9i and 10g. Raw Devices are difficult to manage and administer but provide high performing shared storage structures. When using the raw devices for data files, redo log files, and control files, it may be necessary to use the local file systems or some sort of network attached file system for writing the archive log files, handling the utl_file_dir files and files supporting the external tables.

The following is an example where raw devices are still in use.  When a RAC database is implemented on Solaris platform with Sun Cluster software, raw devices will have to be used as the shared storage for database files. Of course, if using the Veritas DB edition for Cluster on Solaris platform, the Veritas Cluster File system can be used and this provides more options.

Figure 5.4 shows the various types of Oracle related files located on raw devices and non-raw devices.

Figure 5.4:  Using raw devices for shared storage structures

A raw device, also known as a raw partition, is a disk partition that is not formatted. Applications issue I/O calls to transfer data directly from buffers in the user virtual address space to disk. There is no operating system buffering (e.g., page cache), nor is write-order locking imposed. The I/O transfers are conducted through the character-special device driver. As such, I/O transfers generally must adhere to strict requirements imposed by the device driver such as alignment and I/O size and file offsets.


Raw partitions have several advantages:

* They are not subject to any operating system locking.

* The operating system buffer or cache is bypassed, giving performance gains and reduced memory consumption.

* Multiple systems can be easily shared.

* The application or database system has full control to manipulate the internals of access.

* Historically, the support for asynchronous I/O on UNIX systems was generally limited to raw partitions.

The creation and usage of raw partitions should be carefully planned, even if the creation and administration of the raw volumes is relatively simple with the use of the logical volume manager.

Issues and Difficulties

There are many administrative inconveniences and drawbacks such as:

* The unit of allocation to the database is the entire raw partition. A raw partition cannot be used for multiple tablespaces. A raw partition is not the same as a file system where many files can be created.

* Administrators have to create them with specific sizes. When the databases grow in size, raw partitions cannot be extended. Partitions need to be added to support growing tablespaces. Sometimes there may be limitations on the total number of raw partitions that can be used in the system. Furthermore, there are no database operations that can occur on an individual data file. There is, therefore, no logical benefit from having a tablespace consist of many data files except for those tablespaces that are larger than the maximum Oracle can support in a single file.

* The standard file manipulation commands cannot be used on raw partitions, and therefore on the data files. Commands such as cpio or tar cannot be used for backup purposes. Backup strategy will become more complicated.

* Raw partitions cannot be used for writing the archive logs.

* Administrators need to keep track of the raw volumes with their cryptic naming conventions. However, by using symbolic links, the DBA can reduce the hassles associated with names.

For example, a cryptic name like /dev/rdsk/c8t4d5s4 or a name like /dev/sd/sd001 is an administrative challenge. To alleviate this, administrators often rely on symbolic links to provide logical names that make sense. This, however, substitutes one complexity for another.

In a clustered environment like Linux clusters, it is not guaranteed that the physical devices will have the same device names on different nodes or across reboots of a single node. To solve this problem, manual intervention is needed that increases administration overhead.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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