1.2.3.3 Exadata Smart Flash Log

Exadata Smart Flash Log improves transaction response times and increases overall database throughput for I/O intensive workloads by accelerating performance-critical log write operations.

The time to commit user transactions is very sensitive to the latency of log write operations. In addition, many performance-critical database algorithms, such as space management and index splits, are very sensitive to log write latency.

Although the disk controller has a large battery-backed DRAM cache that can accept writes very quickly, some write operations to disk can still be slow during busy periods when the disk controller cache is occasionally filled with blocks that have not been written to disk. Noticeable performance issues can arise even with relatively few slow redo log write operations.

Exadata Smart Flash Log reduces the average latency for performance-sensitive redo log write I/O operations, thereby eliminating performance bottlenecks that may occur due to slow redo log writes. Exadata Smart Flash Log removes latency spikes by simultaneously performing redo log writes to two media devices. The redo write is acknowledged as soon as the first write to either media device completes.

Prior to Oracle Exadata System Software release 20.1, Exadata Smart Flash Log performs simultaneous writes to disk and flash storage. With this configuration, Exadata Smart Flash Log improves average log write latency and increases overall database throughput. But, because all log writes must eventually persist to disk, this configuration is limited by the overall disk throughput, and provides little relief for applications that are constrained by disk throughput.

Oracle Exadata System Software release 20.1 adds a further optimization, known as Smart Flash Log Write-Back, that uses Exadata Smart Flash Cache in Write-Back mode instead of disk storage. With this configuration, Exadata Smart Flash Log improves average log write latency and overall log write throughput to eliminate logging bottlenecks for demanding throughput-intensive applications.