Of Data Guard and the Night Watch; a Date with Chris at the Rijksmuseum

Last Thursday evening was special. On the occasion of Chris Date’s visit to The Netherlands, the product marketing department of Oracle Netherlands had gathered some 25 odd (could also be spelled with dash 25-odd) Oracle experts at The Rijksmuseum in Amsterdam. This remarkable group represented over four centuries of specialist Oracle knowledge; it included for example two members of the Oak Table society.

The evening started with drinks, followed by a tour of the museum. Have you read the Da Vinci Code by Dan Brown? What he describes about the Louvre at night time came all back to me as we walked through a deserted Rijksmuseum. An art historian explained many details about Rembrandt’s The Nightwatch, Vermeer’s Milkmaid, Loveletter and The Little Street. Wel also discussed more works by Jan Steen, Frans Hals, Rembrandt and Heda: Still-life with Gilt Goblet by Willem Claesz. Heda. After this tour we set down to an equisitely laid table in one of the exposition rooms in the museum with walls covered with paintings. When you got up to go the bathroom the very first thing you encountered in the next room was, once again, ! It was very special to have such a personal encounter – not minding the security guard present at all times – with one of the world’s most famous paintings.
Conversation during dinner was almost exclusively dedicated to Oracle technology. I was seated next to Carel Jan Engel for most of the evening: he is a staunch supported of Oracle Data Guard. With over 30 installations to date he may well be one of the world’s most seasoned experts on Data Guard. Someone also told – off the record as there are some legal issues – some horror stories about EMC. Not only does EMC throw incredible amounts of memory at their cache – used to synchronize disks – they also synchronize far more data for each write operation than necessary. A more logical clustering through Oracle Grid Real Application Clusters (complemented by a stand-by database with properly set delay time) is a far leaner and inherently safer solution than low-level diskmirroring. This was topped off with a story about EMC staff upgrading firmware which effectively corrupted several databases.

All in all, a strong case was made for Data Guard. Effectively available since 8.1.7, Data Guard provides for a stand-by database, based on redo-log data that is immediately sent from the primary database (or database cluster as Data Guard is perfectly suited to complement a RAC or GRID) to the stand-by database. There virtually no intrusion on the primay (database). It can be useful to specify a delay: not apply the changes from the primary immediately at the secondary. Carel Jan told about a case where a four hour delay helped to save the day when at the primary someone made an error with huge impact. Since this error had not yet been applied at the stand by database, it was easy to recover the primary using the secondary.

Carel Jan indicated that Data Guard in logical stand-by mode is not robust and reliable enough for a real stand-by database. For maintaining a secondary database for reporting purposes for example it will be sufficient. It is to some extent comparable to replication solutions based on Oracle Streams. However, Streams are intrusive on the primary: scanning the redo-log files takes place at the primary database and that is where messages to consumers are sent. In this respect, Data Guard is far less imposing on the primary database. You might even consider implementing asynchronous data distribution using Oracle Streams on a logical Stand-By database maintained from the Primary by Data Guard.

Other subjects for discussion were Oracle Collaboration Suite – doing okay, can’t complain said one Oracle spokesperson, not abundantly enthousiastic – the future of Oracle tools, the imminent seminar by Tom Kyte in The Netherlands, instrumentation for Oracle DBAs and in particular a tool from Veritas: InDepth that is perfect for identifying performance hotspots across all layers of the application (from Web Brower to Database).

After a three course dinner and a four hour discussion on fairly geeky, techie, nerdie stuff, we were ushered out of this most impressive setting for any Oracle event I have attended!