Exadata Smart Flash Cache transparently caches frequently-accessed data to fast solid-state storage, improving query response times and throughput. Write operations serviced by flash instead of by disk are referred to as write-back flash cache. Write-back flash cache allows 20 times more write I/Os per second on X3 systems, and 10 times more on X2 systems. The larger flash capacity on X3 systems means that almost all writes are serviced by flash.

An active data block can remain in write-back flash cache for months or years. Blocks that have not been read recently only keep the primary copy in cache. All data is copied to disk, as needed. This provides for smart usage of the premium flash space.

If there is a problem with the flash cache, then the operations transparently fail over to the mirrored copies on flash. No user intervention is required. The data on flash is mirrored based on their allocation units. This means the amount of data written is proportional to the lost cache size, not the disk size.

• Exadata Smart Flash Cache Persistent After Cell Restart