Database management systems generally run on only one server at a time. Without sophisticated solutions such as Oracle’s Real Application Clusters, databases must be manually moved between servers. This limitation has led to a situation in which huge amounts of computer resources are wasted because it's difficult to balance server load at the database level.
From SAN to DAN
While SAN (Storage Area Networks) and dynamic application servers have been highly successful for Web and application server load balancing, the database layer is still a serious system bottleneck. SAN has made it easy to move disk storage from server to server, allowing the IT staff to dynamically relocate disk storage as needed (Figure A).
| Figure A |
 |
| The current flexible architecture |
SAN technology allows single-threaded sharing of data by many computers, but a problem remains: how to intelligently relocate databases to different servers when processing demands change. This is where DAN (Database Area Network) technology enters the picture.
The DAN architecture uses a database switch with the underlying SAN to allow databases to move from server to server without affecting availability. Figure B shows both architectures.
| Figure B |
 |
| SAN and DAN architecture |
Database server load balancing is complex and problematic, and many companies waste millions of dollars each year by overallocating database server resources. Worse yet, many shops underallocate server resources, forcing end users to endure unacceptable response times until the database administrator (DBA) can manually relocate the database to a larger server. In order to address this inefficient use of database server resources, researchers are exploring DAN techniques to allow for the dynamic relocation of databases when the processing demands exceed the processing capacity of the server.
3%
2%
