1.2.3.2 Write-Back Flash Cache

Write-Back Flash Cache enables write I/Os directly to Exadata Smart Flash Cache.

With Exadata Smart Flash Cache in Write-Back mode, database writes go to Flash Cache first and later to disk. Write-Back mode was introduced with Oracle Exadata System Software release 11.2.3.2.0.

Write-intensive applications can benefit significantly from Write-Back mode by taking advantage of the fast latencies provided by flash. If your application writes intensively and if you experience high I/O latency or significant waits for free buffer waits, then you should consider using Write-Back Flash Cache.

With Exadata Smart Flash Cache in Write-Back mode, the total amount of disk I/O also reduces when the cache absorbs multiple writes to the same block before writing it to disk. The saved I/O bandwidth can be used to increase the application throughput or service other workloads.

However, if a flash device fails while using Write-Back mode, data that is not yet persistent to disk is lost and must be recovered from a mirror copy. For this reason, Write-Back mode is recommended in conjunction with using high redundancy (triple mirroring) to protect the database files.

The contents of the Write-Back Flash Cache is persisted across reboots, eliminating any warm-up time needed to populate the cache.