Oracle Grid Computing with RAC  Preparing Shared Storage Oracle Tips by Burleson

10g Grid Computing with RAC
Preparing Shared Storage

Raw Devices

Raw Devices have been in use for a very long 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 you use the raw devices for data files, redo log files and control files, you may have 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.

Let’s see an example where raw devices are still in use.  When you implement the RAC database on Solaris platform with Sun Cluster software, you will have to use the raw devices as the shared storage for database files.  Of course, if you use the Veritas DB edition for Cluster on Solaris platform, you will be able to use the Veritas Cluster File system and you have more options.

Fig. 5.4 shows the mix of 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.

Advantages

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. We cannot use a raw partition for multiple tablespaces.  A raw partition is not the same as a file system where we can create many files.
   

• Administrators have to create them with specific sizes.  When the databases grow in size, raw partitions cannot be extended.  We need to add extra partitions to support the growing tablespace.  Sometimes we may have limitations on the total number of raw partitions we can use 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 consisting of many data files except for those tablespaces that are larger than the maximum Oracle can support in a single file.

• We cannot use the standard file manipulation commands on the raw partitions, and thus, on the data files.  We cannot use commands such as cpio or tar 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 the symbolic links, we 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, which will increase administration overhead.

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