Expert Oracle Exadata - Parent Directory

CHAPTER 13 MIGRATING TO EXADATA• If replication breaks, fixing it can be messy. It is often easier to scrap the target andrebuild it.• SQL Apply is more prone to falling behind the source than Golden Gate.• Streams tends to require much more CPU resources than Golden Gate.Advantages of Streams over Golden Gate:• Streams is included with your database license, while Golden Gate is soldseparately• If the source database is Oracle 11.2 (or higher), Streams supports capturingchanges to compressed source tables (basic and OLTP compression). If both thesource and target databases are running Oracle version 11.2.0.2 or higher, then theStreams supports capturing changes to HCC compressed tables too.• All process information is stored in the database, providing convenient remoteaccess.• There is integration with Grid Control.So which should you use, Streams or Golden Gate? We’ve spoken with clients and colleagues whohave used both products. In every case they are using the product for simple replication to a remote sitemuch like you would for database migration. The feedback we get is overwhelmingly in favor of GoldenGate. These users cite Golden Gate’s ease of use, stability, and performance as the most importantreasons. In addition to this, Golden Gate supports non-Oracle database sources. Streams runs inside theOracle database and has not been ported to any other database platform. As mentioned, Streams isincluded in your Oracle database license. Golden Gate is not, and the cost based on the total number ofcores on the source and target servers is not trivial. If you have need for data replication beyond yourExadata migration, then the investment may be well worth it. If you are faced with a one-time migration,Streams may be a better fit for cost.When to Use Streams or Golden GateStreams and Golden Gate both provide a lot of flexibility with near-zero downtime. They are well suitedfor migrations that require changes to the target tables. This includes resizing table and index extents,implementing partitioning strategies, and compressing tables. They also allow you to change schemas,tablespaces, and disk groups, among other things.What to Watch Out for with Streams and Golden GateSince Streams and Golden Gate propagate database changes by mining the redo logs, you must accountfor NOLOGGING operations. This can be done at the tablespace level using alter tablespace forcelogging or at the database level using alter database force logging.Here are a few examples of NOLOGGING operations:• CREATE TABLE newtab NOLOGGING AS select * from …)• insert /*+ append */ … (for tables in nologging mode)• Direct path SQL*Loader (for tables in nologging mode)447