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 RAC Using RAW Storage
Oracle Tips by Burleson Consulting

Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Some Oracle files can be written to unformatted disk areas known as raw devices. Some sources may also call these raw volumes, raw partitions, or raw disks. The Oracle files which can be written to raw devices are:

  • OCR

  • Voting disk

  • Datafiles

  • Redo logs

  • Control file

  • SPFILE

It is worth noting that the archive logs and RMAN backups do not make the raw storage list. This is because a raw device can only handle one file at a time. Given a partition with no filesystem, there are three available options: format the partition for a particular filesystem, use the partition in an ASM diskgroup, or use the partition as a raw device on which a single file may be placed.

 

One reason behind the popularity of raw devices is performance.  In the past, raw devices were the only way by which a system could be set up to take advantage of Direct I/O (DIO); that is, I/O that bypasses the filesystem cache.  In fact, Direct I/O has been supported in the ext3 filesystem since Enterprise Linux 2.1.  Support for enhanced Asynchronous I/O (AIO) with Direct I/O was added in Enterprise Linux 4, even when using an ext-based filesystem.  According to Red Hat, ext3 filesystem access with AIO and DIO can perform within 3% of raw I/O performance.  Direct I/O is also enabled when using OCFS/OCFS2.

 

Note: The filesystemio_options parameter allows a DBA to direct how Oracle will perform I/O.  A setting of ‘directio’ will allow Direct I/O access.  ‘asynch’ allows Asynchronous I/O access.  ‘setall’ allows both.  Consult the OS specific documentation to determine if the system is optimized for both DIO and AIO.

 

In Oracle 11g it is very common to find the OCR and voting disk of a RAC cluster on raw devices.  This is because those two files are very small, very static in size, and cannot be placed in ASM.  However, according to Oracle Metalink (Oracle’s support system), raw device support will be completely unavailable in Oracle 12g.  This may be due to the fact that raw devices have been declared obsolete in Linux since kernel version 2.6.3, and support for raw devices will soon be gone. However, there is no need to fear this change.  Instead, it is only necessary to make room for a few changes in vocabulary.

 

Those familiar with using raw devices on Linux may get a shock when using Redhat Enterprise Linux 5 (RHEL5) or Oracle Enterprise Linux 5 (OEL5) as there appears to be no raw device support.  As mentioned above, in kernel version 2.6.3, this support is officially deprecated.  However, it is still possible to configure a /dev/raw volume using udev rules.

 

In RHEL4 it was possible to simply place entries in /etc/sysconfig/rawdevices which mapped a block device, i.e. /dev/sda1, to a raw device, i.e. /dev/raw1.  Using the raw devices service, the mapping would take effect and /dev/raw would be a usable area.

 

 Note:  In a Windows environment, a raw device is simply a logical partition created in Disk Manager that is not formatted and has no drive letter.

     

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