This statistic does not apply to systems not using Exadata Hardware and the Oracle Database 11g new feature, Database Smart Flash Cache (DSFC).

QUESTION: What is an UnOptimized read?

The UnOptimized read is one that is not serviced by the DSFC or the Exadata Cell Smart Flash Cache (ECSFC). Thus, if you do not have Exadata or do not use the Database Smart Flash Cache, all reads will be default fall into the category of UnOptimized.

So, what are UnOptimized reads?

In Oracle 11g there is now the ability to create a second layer of buffer cache called the DSFC, which is used as an overflow for the buffer cache. It resides on SSD disks that are available to the OS. As data gets aged off of the buffer cache, it is written into the DSFC. It is set up as a data file that resides on SSD. Although this is not nearly as fast as RAM, it is much faster than going to disk. When retrieving data, the SGA is checked first, then the DSFC, and then disk.

In addition to the DSFC which is used for the entire instance, within Exadata each storage cell has its own large amount of flash memory (SSD), called ECSFC, which is used somewhat like a disk cache.

Depending on the setting of the database object cell_flash_cache option, data in the cell flash cache will be handled differently. Using the keep option the storage cell will cache data from objects with that setting in the cell flash cache as long as possible. Objects with cell_flash_cache set to default will cache random reads from the object, but not table scans. An object set to none will never be cached.

Note: This is similar to the Direct Path Read, where in the case of a table scan; the data is neither read into the Database Buffer Cache nor kept in the ECSFC,unless the cell_flash_cache option for that object is set to KEEP.

ANSWER :An UnOptimized read in Oracle 11g is a read that is not found in the DSFC; On Exadata an UnOptimized read is not found in the DSFC or in the ECSFC.

An Optimized read is one that finds its data in either one of those two caches with Exadata or in the database smart flash cache in a non-Exadata system. A read that finds its data in the Database Buffer Cache is not counted as an Optimized or UnOptimized read. The cache reads are categorized as gets in AWR reports.

A read is a physical I/O operation, either to the DSFC(SSD disk) or the disk, or to the ECSFC.

Therefore, on an Oracle 11g AWR report, don’t worry if all of your reads are UnOptimized.