Database Reference
In-Depth Information
4.
Constantly monitor the interconnect traffic and RAC cluster wait events. You can either
monitor these cluster wait events on the Clusterware cache coherence page of Oracle
Enterprise Manager, or check if there are any of the GCS-related wait events shown on the
Top 5 Timed Events of AWR report.
Load Balancing Among RAC Instances
Another important feature related to RAC scalability is designed to distribute the workloads among all RAC instances
for optimizing performance. This workload distribution occurs when a client connects to the RAC database for the
first time or when the client connection is failed over from a failed instance to a surviving instance, in which case the
load balancing works together with the failover feature. Oracle provides two kinds of load balancing: client-side load
balancing and server-side load balancing.
Client-side load balancing is enabled by setting LOAD_BALANCE=yes in the client tnsnames.ora file:
KHDB =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = kr720n1-vip)(PORT = 1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = kr720n2-vip)(PORT = 1521))
(LOAD_BALANCE = yes)
(CONNECT_DATA =
(SERVICE_NAME =
khdb.dblab.com
)
)
)
)
With LOAD_BALANCE enabled, Oracle Net chooses an address to connect based on load balancing
characteristics from pmon rather than on sequential order. This order ensures the even distribution of the number of
user sessions connecting to each database instance. In Oracle 11gR2, the list of addresses has been replaced with one
SCAN entry. If you define SCAN with three IP address in the corporate DNS (Domain Name Service), client-side load
balancing is moved to the DNS level among the three IPs for the SCAN. Chapter 9 gives more details about the 11gR2
SCAN configuration. Some old-version Oracle clients such as pre-11gR2 clients (11gR1, 10gR2, or older) may not be
able to get the benefits of SCAN, as these clients will not be able to handle the three IPs of SCAN; instead, they may
just connect to the first one. If the one that the client connects to fails, the client connection fails. Therefore, it may be
better to use the old way by listing three VIP addresses of the SCAN IPs on the tnames.ora files.
Unlike the selection of the RAC node for the incoming user connection by client-side load balancing, in
server-side load balancing the least-loaded RAC node is selected. Then, by using information from the Load Balancing
Advisory, the best RAC instance that is currently provided to the service is selected for the user to connect. There is no
need for any code change in the application side for server-side load balancing. However, the initialization parameter
remote_listener needs to be set to enable listener connection load balancing. In 11gR2, the remote_listener is set to
SCAN:PORT, as shown in the following example:
SQL> show parameter _listener
NAME TYPE VALUE
---------------------- ------- ------------------------------
local_listener string (DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(
(PROTOCOL=TCP)(HOST=172.16.9.171)
(PORT=1521))))
remote_listener string kr720n-scan:1521
The remote_listener parameter is set by defult if you use DBCA to create the RAC database.
