9.6.16 Write Back Flash Cache on Extreme Flash Cells

On Extreme Flash cells, flash cache runs in write back mode by default, and takes 5 percent of the flash space. Flash cache on Extreme Flash cells is not used as a block cache because user grid disks are already created on flash and therefore caching is not needed. However, flash cache is still useful for the following advanced operations:

• Columnar caching caches Exadata Hybrid Columnar Compression (EHCC) table data on flash cache in a pure columnar layout on an Extreme Flash cell.

• Write I/O latency capping cancels write I/O operations to a temporarily stalled flash, and redirects the write to be logged in the write back flash cache of another healthy flash device on an Extreme Flash cell

• Fast data file creation persists the metadata about the blocks in the write back flash cache, eliminating the actual formatting writes to user grid disks, on an Extreme Flash cell.

Administrators can choose to configure flash cache in write through mode on Extreme Flash cells. Columnar caching works in write through flash cache mode, but write I/O latency capping and fast data file creation require write back flash cache to be enabled.