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Oracle9i RAC
Oracle Real Application Clusters Configuration and Internals
Mike Ault & Madhu Tumma
ISBN 0-9727513-0-0
Publication Date -
June 2003
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Sample Text:
Oracle9i RAC now
supports both the cluster file systems and the raw devices to store the
shared data.
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.
A cluster file
system (CFS) is a file system that may be accessed (read and write) by
all the members in the cluster at the same time. This implies that all
the members of the cluster have the same view. Some of the popular and
widely used cluster file system products for Oracle9i RAC include: HP
Tru64 CFS, Veritas CFS, IBM GPFS, Polyserve Matrix Server, and Oracle
Cluster File system. The cluster file system offers:
-
Simple
management.
-
The use of
Oracle Managed Files with RAC.
-
A Single Oracle
Software Installation.
-
Auto-extend
Enabled on Oracle Data Files.
-
Uniform
accessibility of Archive Logs.
-
ODM compliant
File systems.
In our view, when
the cluster file system is available for a given platform, we should use
the CFS in preference to the raw partitions. With the availability of
the ODM-compliant cluster file systems, using the CFS makes much more
sense. ODM-based CFS offer similar or even better performance over raw
partitions, and far superior administrative convenience. The cluster
file system set of features is rapidly advancing.
We will offer a
detailed explanation of the cluster file system, as well as advances in
usage and functionality, in later sections of the chapter.
If you need to
learn about Oracle9i RAC, Mike Ault and Madhu Tumma's book "Oracle9i
RAC", is the only RAC book on the market. You can order it here:
http://www.rampant-books.com/book_2003_1_rac.htm
Regards,
Don Burleson
www.dba-oracle.com
www.remote-dba.net
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