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Oracle Tips by Burleson |
Attributes of an Oracle RAC
Cluster
The Oracle9i RAC allows for a single or
multi system Image, what this means is that the root and other
system files can either be localized to internal or non-shared disks
for each server in the RAC or each can load the operating system
from a central set of disks.
If the RAC cluster uses a SAN or other
network storage device then this is known as a Shared System Disk
(root) system.
A RAC system must use a cluster file system
where any server can read or write to any disk in the shared disk
subsystem. This allows access to all datafiles, control files and
redo and rollback areas by any instance. This ability to access all
disks allows for instance recovery after an instance failure has
occurred. The work of the failed instance is automatically absorbed
by all surviving nodes until the failed instance is brought back
online, at which time it is fully synchronized and restored to
service automatically.
A RAC cluster provides for automatic shared
execution of oracle applications. This means that for any Oracle
instance application all queries and other processing is
automatically shared among all of the servers in the RAC cluster.
The sharing of application processing to all
servers in the RAC cluster leads to automatic load balancing across
all cluster members.
The ability of a RAC cluster to provide
shared application execution and automatic load balancing leads to
the true scalability of applications without code or data changes.
One limit on shared-none or federated
databases was the amount of time required for failover. Due to the
requirements for rebuild or rehash of data fail over time in a
non-shared disk environment can be prohibitive. In a RAC cluster
fail over time is virtually null since oracle handles the
redistribution of work automatically and no rehash of data is
required since the same data files are shared by all participating
nodes in the RAC cluster.
The number of nodes in a RAC cluster is
basically only limited by the number of servers that your LAN
architecture allows to connect into the disk farm. In the SQL Server
2000, in the cluster database you have a hard limit of 2 or 4
servers depending on what level of server license for Windows 2000
you have loaded on your server. The convoy effect limits the number
of servers that an IBM type structure allows. Due to the cache
fusion and the high speed cluster interconnect the Oracle9i RAC
allows for a virtually unlimited number of nodes in a RAC cluster.
Oracle9i RAC allows for TCP/IP/ UDP/ VIA for
the protocol used by the cluster interconnect. The speed of the
interconnect should be at a minimum of 100 mbit/sec. The maximum
number of instances that can be interconnected is OS platform
dependent. He distance allowed between nodes on a cluster
interconnect is system and LAN specific.
Oracle RAC displays great disaster
tolerance. If a single or multiple nodes fail, the load is
redistributed between all of the remaining nodes. If the database is
globally distributed then a disaster to a single site will not
effect other sites.
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