||Donald K. Burleson
Oracle RAC Tips
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.
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 datafile. 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.
We cannot use the standard file manipulation
commands on the raw partitions, and therefore 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 that increases administration overhead.
For more information,
see the book
Grid and Real Application Clusters 30% off if you
buy it directly from Rampant TechPress .
Written by top Oracle
experts, this RAC book has a complete online code depot with
ready to use RAC scripts.