7.3.4 Enhanced OLTP High Availability During Cell Outages and Flash Failures

Oracle Exadata System Software release 19.1.0 enhances application performance after the failure of a flash device or storage server by partially duplicating hot data in the flash cache of multiple storage servers.

If a cell goes offline or a flash device fails, then the databases redirect IOs to use secondary mirrors for data read operations because the primary mirrors are unavailable due to the outage. However, the secondary mirrors might not be in the flash cache of the respective cell, so the databases must read the data from hard disks. This can negatively impact application performance due to flash cache misses.

With this feature, Oracle Exadata System Software prefetches the secondary mirrors of the OLTP data that is most frequently accessed into the flash cache. This prefetching is done as a background task. Oracle Exadata System Software automatically manages the secondary mirrors in the flash cache in an optimal way so that newer or more active secondary mirrors replace the cold data in the cache. In addition, when a flash device is replaced, this feature also ensures that the newly replaced flash device is made more current (warmed up) before redirecting database read operations to the new flash device. Thus, this feature provides better high availability for application performance by greatly reducing the secondary mirror flash cache misses during cell or flash device failures and flash device replacements.

This feature is useful for OLTP workloads only. Oracle Exadata System Software does not cache the secondary mirrors for scan data. Also, this feature is only enabled for write-back Flash Cache. No secondary mirror caching is done for write-through Flash Cache.

Minimum requirements:

• Oracle Exadata System Software release 19.1.0
• Oracle Database 19c
• Exadata Write Back Flash Cache on High Capacity storage servers
• Exadata Database Machine X6 or later (due to flash cache size requirements)