Undo Tablespaces in a RAC Environment
As with redo logs, each instance in a RAC environment must have its own undo tablespace on
a shared drive or disk group. This undo tablespace is used for rolling back transactions during
normal transactional operations or during instance recovery. In addition, the undo tablespace is
used by other nodes in the cluster to support read consistency for transactions that are reading
rows from a table on node rac2 while a data-entry process on node rac1 makes updates to the
same table and has not yet committed the transaction. The user on rac2 needs to see the beforeimage
data stored in rac1??™s undo tablespace. This is why all undo tablespaces must be visible to
all nodes in the cluster.
Failover Scenarios and TAF
If you have configured your client correctly and the instance to which the client is connected to
fails, the client connection is rapidly switched to another instance in the cluster and processing
can continue with only a slight delay in response time.
Here is the tnsnames entry for the service racsvc I created earlier:
racsvc =
(description =
(address = (protocol = tcp)(host = voc1)(port = 1521))
(address = (protocol = tcp)(host = voc2)(port = 1521))
(address = (protocol = tcp)(host = voc3)(port = 1521))
(load_balance = yes)
(connect_data =
(server = dedicated)
384 Oracle Database 11g DBA Handbook
(service_name = racsvc.world)
(failover_mode =
(type = select)
(method = basic)
(retries = 180)
(delay = 5)
)
)
)
I will show you what happens and how you will know if a session is connected to the cluster
and its instance fails.
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