How the Optical Subsystem Affects Performance

The Optical Subsystem extends the storage capabilities of the database server for simple large objects (TEXT or BYTE data) to write-once-read-many (WORM) optical subsystems. The database server uses a cache in memory to buffer initial TEXT or BYTE data pages requested from the Optical Subsystem. The memory cache is a common storage area. The database server adds simple large objects requested by any application to the memory cache as long as the cache has space. To free space in the memory cache, the application must release the TEXT or BYTE data that it is using.

A significant performance advantage occurs when you retrieve TEXT or BYTE data directly into memory instead of buffering that data on disk. Therefore, proper cache sizing is important when you use the Optical Subsystem. You specify the total amount of space available in the memory cache with the OPCACHEMAX configuration parameter. Applications indicate that they require access to a portion of the memory cache when they set the INFORMIXOPCACHE environment variable. For details, see INFORMIXOPCACHE.

Simple large objects that cannot fit entirely into the space that remains in the cache are stored in the blobspace that the STAGEBLOB configuration parameter names. This staging area acts as a secondary cache on disk for blobpages that are retrieved from the Optical Subsystem. Simple large objects that are retrieved from the Optical Subsystem are held in the staging area until the transactions that requested them are complete.

The database server administrator creates the staging-area blobspace in one of the following ways:

• With ON–Monitor (UNIX only)
• With the ISA utility
• With the onspaces utility

You can use onstat -O or ISA (Performance > Cache > Optical Cache) to monitor utilization of the memory cache and STAGEBLOB blobspace. If contention develops for the memory cache, increase the value listed in the configuration file for OPCACHEMAX. (The new value takes effect the next time that the database server initializes shared memory.) For a complete description of the Optical Subsystem, see the IBM Informix: Optical Subsystem Guide.