SAND CDBMS Administration Guide
Virtual Mode Operations

Optimizing Virtual Mode Performance

Virtual Mode operations can put stress on the I/O subsystem. For best performance, the Delta File should reside on a local file system (not a Network File System) and, if possible, not on the DASD device used for the real database file. To reduce contention for the DASD device, and to avoid queuing a large number of I/O operations for execution, the database should be defined with multiple file locations (drives). These file locations should be defined to improve parallelism by spreading out I/O.

Determining the appropriate maximum Delta File size requires analysis of the specific situation at hand. As with the Cache value, the size of the Delta File is directly affected by the nature of the database and the query. For a small database (that is, less than 20 GB), a MaxDeltaFileSize value of 1024 MB (1 GB) will be sufficient. For a large database, with which very large intermediate result sets can be created, a limit based on a relative size of MaxDeltaFileRatio=10 (that is 1/10 of the actual database size) is a safe starting point.