Raw device is proper solution to prevent disk I/O of Oracle Database Server?

by / March, 2011 / Published in Oracle DBA Tips

disk io tuning

Raw device or raw disk is an unformatted disk partition. Raw devices do not have a file system mounted on them, and they are written to and read from character device drivers. Due to the absence of a regular file system such as JFS, UFS or NTFS, such partitions are considered to be raw or "uncooked". When a regular file system is used, the file system manages all I/O requests, each of which passes through the file system buffers. In other words, data is written to the file system buffers and then synchronized with the physical disks. When an Oracle file such as data file, online redo log files, control files are created on raw disk, Oracle takes responsibility for reading from and writing to that disk, since the file system in absent.

Unix uses device files to communicate with hardware components such as disk drives and tape drives. Each disk drive has corresponding entry in the /dev directory. A cooked disk may have both a block device file and a character device file associated with it. However, a raw disk has only character device file. A character device file uses its own device drivers to read from and write to a disk and performs I/O operations in variable sizes. A block device file is dependent on file system buffers for performing I/O operations, and it reads from and writes to a disk in fixed-length size.

Raw devices help performance, but only disk I/O is major bottleneck. Oracle's stylish advance memory architecture uses data block buffers to store and manage data reasonably well and minimize disk I/O. If you found that most I/O bottleneck would not have been avoided if raw device had been used. Instead, the bottlenecks occur because far too few disks or controllers were available for optimal data placement or even because the data files were not optimally spread across all available disks and controllers. Raw devices are not a magic potion to prevent I/O bottlenecks. Whether you use raw devices or file systems, an optimal file placement strategy is the right approach to avoid such I/O bottleneck or I/O contentions.

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Indeed, using raw device we can get better performance due to direct reads and writes enable. This direct read and write requests skipped file system buffers. Directly writing to or reading from disk eliminates the work of synchronizing the disks with the file system buffer cache and vice versa. This is good for a pure OLTP database systems, in which the reads and writes are so random that once data is written or read it won't be needed for a while.

Nowadays, we can abolish file system buffer cache and able to enable direct read and write on regular file system. But we can't get pure direct I/O because some levels of buffers are relevant in file system even enables direct I/O option. This feature is only with Raw Devices only. In raw devices we can say that no direct read I/O or write I/O performing.

More memory utilization may occur in regular file systems because Oracle itself consumes memory and file system buffers also consumes memory from Database server. These things indicate more memory utilization in Oracle Database Server. In present scenario, it is not problematic because we got higher server configuration and more memory in low cost budget.

Major disadvantage of raw device is management of raw devices. It is not easy to maintain and manage raw devices in Database server. With regular file system, this task is relatively easy to manage. To manage raw devices partitions you need expertise and extra skill sets. It means, remote dba experts can manage this typical servers.

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