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Oracle 9i Database Scalability Enhancements
This is one of a series of articles on the new features of Oracle 9i and their impact on business and technical issues .
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The biggest change in terms of scalability in Oracle 9i is undoubtedly the introduction of Real Application Clusters
for Windows, Linux and UNIX. This feature provides load-balancing, scalability and hot-standby all rolled into one
without requiring any application redesign or database shutdown to add more resources to the cluster.
The enhancement of resource management to include undo pool, execution time and active session quotas is also
useful for heavily used systems as is the reduction in resources required to support a user.
Let’s look at these new features in more detail.
Oracle 9i Real Application Clusters
Oracle have always boasted of the ability of their databases to support
large numbers of users, but scalability is more than just being able to
support many users. It is also a case of how easy it is to increase the
number of users that can be supported.
Oracle 9i has performance enhancements that reduce resource
requirements but there is also the need to be able to add more
resources to support more users and to provide redundancy in case of
failure. Real Application Clusters allow exactly that, thereby making
scalability simply a case of adding more servers to the cluster.
Oracle 7 and 8 supported a form of clustering called Oracle parallel
server, but this was complex to implement because the application had to be aware that it was running in a parallel
Real Application Clusters in Oracle 9i are a replacement for and an enhancement of the previous parallel server
technology. Applications no longer have to be aware that they are running in a cluster and new servers can be added to
or removed from the cluster at any time without having to shut down the database at all.
The major business benefits of Real Application Clusters (RAC) are :
scalability on demand - extra servers and disks can be added as and when required
low cost of ownership - RAC can be implemented using low-cost Intel servers running Linux
high availability included - each server in the cluster acts as a hot standby for every other server and "fail over"
can occur in less than one minute
transparent to the application - applications need not be aware of the cluster, everything is handled by the
database mangement software
In technical terms the new features include a new "cache fusion architecture" which as the name suggests treats the
memory caches of all the servers in the cluster as a single cache. This means that reads from disk only have to take
place when none of the servers has the requested data in memory. If one of the servers does have the required data in
memory, it is transferred across the high-speed bus connecting the servers, to the memory of the server that made the
Other RAC-related new features include :
enhancement of Oracle Enterprise Manager to be able to start and shut down RACs
extension of automatic undo management to the whole cluster
Scalable Session State Management
The amount of resources required by Oracle 9i databases for each user has been reduced by improvements to the
network layer, shared (multithreaded) server enhancements, improvements for the support of Java sessions and
shared memory improvements. Together these improvements allow more users to be supported on the same hardware.
Automatic Resource Management
Oracle 8i introduced the facility to allocate resource quotas to various groups of users (known as consumers) by use of
the database resource manager. This enhanced the ability of the dba to share resources appropriately amongst the users
by giving them the ability to manage mixed workloads (e.g. reporting and transaction processing) and to control
system performance. The resources that could be controlled were the percentage of CPU time, the degree of
parallelism and priorities
Oracle 9i improves on this by providing the dba with more granular control over resources, adding the following new
resource plan directives:
undo pool quotas
active session quotas
estimated execution time quotas
automatic consumer group switching
Undo Pool Quota
The undo pool quota controls the maximum amount of undo allowed for a consumer group. When the limit is reached,
the active DML statement is terminated and no other members of the consumer group can perform any DML until
undo space is released from the pool by other sessions within the group or the dba manually increases the undo quota
for the group.
Active Session Quota
This directive controls the maximum number of active sessions allowed for a consumer group. When the limit is
reached, new sessions can be queued until a session becomes available. A time-out period can also be specified to
determine the maximum length of time a new session will wait until it is aborted with an error.
Estimated Execution Time Quota
This enables the dba to set an execution-time limit for an operation. If the database estimates that an operation will
exceed this limit, the operation is aborted and an error returned to the user. This error can be trapped and the operation
rescheduled if needed.
Automatic Consumer Group Switching
Analogous to the execution time quota is the ability to specify that a session should be switched automatically to a
different consumer group if it is active (i.e. running and consuming resources, not idle) for more than the specified
time. This switch will occur even if the active session quota for the new group has already been reached. One the
previously switched session becomes idle again, it is switched back to its original consumer group. The session can
also be switched prior to starting execution if the database estimates that the operation will take longer than the
specified time limit and the use of estimates is specified.
See the Oracle9i Database Administrator's Guide for more details on using these features or see our overview of the
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