5.1.3.4 About Flash Cache Management in IORM Plans

I/O Resource Management can provide predictable performance by guaranteeing space in Exadata Smart Flash Cache.

With multiple databases and pluggable databases (PDBs) sharing the storage, flash cache space becomes a critical resource that requires management. IORM can reserve space for critical databases or PDBs while preventing less important or rogue entities from consuming the entire flash cache.

You can use the following attributes in the interdatabase plan to set limits for flash cache resources. A hard limit means that the specified limit cannot be exceeded even when the cache is not full. A soft limit means that the specified limit can be exceeded if there are available resources.

• flashCacheMin — Specifies the minimum amount of flash cache space that is guaranteed for the specified database, even if the blocks are cold. This is a hard limit.

  Because flashCacheMin is a guaranteed reservation, the sum of flashCacheMin across all directives should be less than the size of the flash cache to ensure that each database gets its respective quota.

• flashCacheLimit — Specifies the soft maximum amount of flash cache space that is available to the database. If the flash cache is not full, a database can exceed the flashCacheLimit value.

• flashCacheSize — Specifies the hard maximum amount of flash cache space that is available to the database. The flashCacheSize value cannot be exceeded at any time.

  However, if you set flashCacheSize to a value that is lower than the current space occupied by the database, then starting with Oracle Exadata System Software release 20.1.0 excess data is proactively cleared from the cache. Previously, excess data was only removed when overwritten by other data.

  Starting with Oracle Exadata System Software release 19.2.0, flashCacheSize is not a guaranteed reservation if the sum of the flashCacheSize across all directives is more than the size of the flash cache. In this case, you can also specify flashCacheMin to define a guaranteed minimum quota.

Within each database, you can manage the minimum and maximum quotas for PDBs with Oracle Database Resource Manager, in a container database (CDB) resource plan using the memory_min and memory_limit directives.

Cache limits in an interdatabase IORM plan directive constrain the settings in any corresponding CDB plan. Consequently, if an interdatabase IORM plan directive specifies flashcachemin and flashcachesize settings for a database, then the PDB-specific memory_min quotas in the CDB plan represent fractions of the flashcachemin setting, and the PDB-specific memory_limit values represent fractions of the flashcachesize.

However, if the interdatabase IORM plan directive specifies flashcachesize without flashcachemin, then the PDB-specific memory_min settings are ignored while the memory_limit settings continue to represent fractions of the flashcachesize.