RAC is a complex system to implement.
In the authors’ experience, most
companies require a consultant to come in to
help plan the move to RAC and for the actual
There are many different pieces to the
RAC environment from networking to disk drives
to clusterware to Oracle itself.
On top of that, there are some costly
In order to implement a RAC system, a shared
storage device is required.
A single instance database can use Direct
Attached Storage (DAS), which is an array of
inexpensive disks connected to a single server.
A SAN (Storage Area Network) is much more
expensive and is capable of connecting to many
servers, usually through fibre-channel
This requires a unique set of hardware
ranging from Host Bus Adapters
(HBA), a fabric switch,
to the SAN itself, and it can get very costly.
Redundancy can also be costly.
Most administrators require redundancy
within each server as well.
This means doubling up on hardware, and
doubling the hardware equals double the cost.
For example, multiple Host Bus Adapters,
multiple network cards, multiple power sources
and more can be required.
The multiple HBA cards are used in case a
single one fails, but this usually requires
expensive software to manage.
Another cost is the network connection.
The RAC system requires a cluster
interconnect in order to accommodate RAM-to-RAM
transfers of data blocks.
This interconnect must be very fast, high
bandwidth with low latency.
Interconnects, such as InfiniBand
and Myrinet, can accommodate this but are very
Though RAC does provide horizontal
scalability, if the cluster interconnect cannot
handle the traffic, extra servers will actually
degrade performance instead of helping it.
The only way around this issue is to
change the entire application to accommodate
RAC, or purchase other means of disk storage
such as Solid State Disk.
There is a definite learning curve when it comes
Because of all the different components
that make up a RAC environment, multiple levels
of training may be required.
System Administrators will have to learn how to
work with the disk resources.
Complex SAN environments such as EMC and
NetApp can require training of their own.
In addition, Oracle RAC will only
function when using specific disk setups such as
ASM, OCFS, or a 3rd Party CFS, and the
administrator will have to assist in setup.
Setting up and administering the hardware
is no small task!
Network Administrators will have to learn how to
work with the new interconnect.
If a specialized interconnect such as
used, training and consulting may be required.
Of all the staff, DBAs will have the greatest
They will have to understand how to set
up and administer clusterware, volume manager or
the file system of choice, the RAC specific
features of Oracle, and troubleshooting for
While this does not sound like much, it
makes up many days of training, lots of trial
and error, and even a little bit of “miracle
work” at times.
As the manager, one may require training to deal
with setting up training sessions, consulting,
and dealing with employees with some great new
marks on their resumes!