6.3.1 Monitoring Exadata Smart Flash Cache

Exadata Smart Flash Cache holds frequently accessed data in flash storage, while most data is kept in very cost-effective disk storage. Caching occurs automatically and requires no user or administrator effort. Exadata Smart Flash Cache can intelligently determine the data that is most useful to cache based on data usage, access patterns, and hints from the database that indicate the type of data being accessed.

Exadata Smart Flash Cache accelerates OLTP performance by providing fast I/O for frequently accessed data blocks. It also accelerates Decision Support System (DSS) performance by automatically caching frequently scanned data and temporary segments, providing columnar cache storage, and enabling other features that optimize the performance of large analytic queries and large loads.

Exadata Smart Flash Cache can operate in Write-Through mode or Write-Back mode. In Write-Through mode, database writes go to disk first and subsequently populate Flash Cache. If a flash device fails with Exadata Smart Flash Cache operating in Write-Through mode, there is no data loss because the data is already on disk.

In Write-Back mode, database writes go to Flash Cache first and later to disk. The contents of the Write-Back Flash Cache is persisted across reboots, eliminating any warm-up time needed to populate the cache. Write-intensive applications can benefit from Write-Back caching by taking advantage of the fast latencies provided by flash. The amount of disk I/O also reduces when the cache absorbs multiple writes to the same block before writing it to disk. 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.

On Extreme Flash (EF) storage servers, all of the data resides in flash. Consequently, Exadata Smart Flash Cache is not required for normal caching. However, in this case Exadata Smart Flash Cache is still used to host the Columnar Cache, which caches data in columnar format and optimizes various analytical queries.

Performance issues related to Exadata Smart Flash Cache typically exhibit increased cell single block physical read latencies in the database. However, occasional long latencies associated with cell single block physical read do not necessarily indicate a performance issue with Exadata Smart Flash Cache, and may simply indicate that the read request was not satisfied using Exadata Smart Flash Cache.

Oracle Exadata System Software release 25.2.0 introduces enhancements to the caching algorithms, enabling a storage server to perform a partner read to a remote cache rather than a more expensive disk read on the local server. This capability effectively transforms a local flash cache miss into a remote cache hit, improving read I/O performance and minimizing disk reads. Partner reads may be satisfied by using mirrored data in either the XRMEM cache or Exadata Smart Flash Cache of the partner storage server. In either case, a successful partner read also counts as a flash cache hit on the primary storage server.

Parent topic: Monitoring Oracle Exadata System Software Components