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Oracle Tips by Burleson |
Benefits of RAC
In the beginning was Oracle Parallel Server
or OPS, now we have Real Application
Clusters or RAC. It is more than just a name
change. RAC provides a process known asw cache-fusion which allows
direct sharing of Oracle database blocks between cache areas of
Oracle instances participating in RAC via a high speed interconnect.
This direct sharing eliminates the biggest performance robbers in
the OPS architecture, the DB block PING.
A DB block PING would occur when an instance
participating in an OPS database would have a block in its cache
that another participating instance required. In OPS if another
instance required the block in the cache of a second instance, the
block would have to written out to disk, the locks transferred and
then the block re-read into the requesting instance. As you can
imagine all of this IO was expensive in terms of time and
performance.
The new RAC with cache fusion exploits the
new high speed buses and architectures available in the new servers.
By eliminating the blockage caused by pinging, RA enables better,
more efficient scaling. In order to add more processing or user
capacity you simply add a server to the LAN, load the appropriate
software and Oracle9i and you are up and operating.
What Is a DB Cluster?
A DB cluster consists of a group of
independent servers, connected via a LAN
that share a set of disk assets via the LAN
and have a high speed cluster interconnect that allows connection of
their processors to the other processors in the cluster. The
clusters do not share memory and act as independent servers. The
servers can either be single processor or SMP (Symmetric
Multi-Processor) nodes. A DB cluster provides fault-tolerance and
allow for modular growth. By having the built in redundancies of
multiple servers the chance that a single point of failure will
prevent database access is unlikely.
DB Cluster Architectures
Essentially there are only two basic
architectures used in DB clusters, these are the shared-nothing and
shared-disk. The shared-nothing cluster uses dedicated disk assets
for each server in the cluster. The shared-disk cluster uses a SAN
type storage array to serve the cluster.
Examples of systems that use the shared
nothing architecture are the IBM RS/6000 SP and the federated
database approach used by SQL Server. This architecture is rapidly
losing ground to the shared-disk architecture.
The shared-disk architecture is being
adopted almost 100% by IBM, SUN and HP and other vendors. It offers
better redundancy, and , better connectivity through technologies
such as fiber-channel, hub and shared SCSI.
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