10.5.1 Oracle Exadata Smart Flash Log

The time to commit user transactions is very sensitive to the latency of log writes. In addition, many performance-critical database algorithms, such as space management and index splits, are sensitive to log write latency. Oracle Exadata Storage Server Software speeds up log writes using battery-backed DRAM cache in the disk controller. Writes to the disk controller cache are normally very fast, but they can become slow during periods of high disk I/O. Oracle Exadata Smart Flash Log takes advantage of flash memory in Exadata Storage Server to accelerate log writes.

Flash memory has very good average write latency, but it has occasional slow outliers that are one to two orders of magnitude slower than the average. Oracle Exadata Smart Flash Log performs redo writes simultaneously to both flash memory and the disk controller cache, and completes the write when the first of the two completes. This improves the user transaction response time, and increases overall database throughput for I/O intensive workloads.

Oracle Exadata Smart Flash Log only uses Exadata flash storage for temporary storage of redo log data. By default, Oracle Exadata Smart Flash Log uses 32 MB on each flash disk, for a total of 512 MB across each Exadata Cell. It is automatically configured and enabled. No additional configuration is needed.