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

You can use the following attributes 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 in an interdatabase IORM plan directive. This is a hard limit that guarantees space, even if the cached data is cold.

  Starting with Oracle Exadata System Software release 26.1.0, flashCacheMin is also available as an attribute in a cluster plan directive, where it specifies the minimum amount of flash cache space that is guaranteed for the corresponding cluster.

  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 entity gets its respective quota.

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

  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.

  Starting with Oracle Exadata System Software release 20.1.0, if you set flashCacheSize to a value that is lower than the current space occupied by the database, 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 26.1.0, flashCacheSize is also available as an attribute in a cluster plan directive, where it specifies the hard maximum amount of flash cache space that is available to the corresponding cluster.

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

  Note:

  Starting with Oracle Exadata System Software release 26.1.0, the flashCacheLimit attribute is deprecated, and a warning message is issued when flashCacheLimit is set.

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.

Also, to ensure the minimum amount of flash cache space for a CDB, you must specify flashcachemin in the interdatabase IORM plan and PDB-specific memory_min settings in the corresponding CDB resource plan. If the CDB resource plan is not specified or does not contain PDB-specific memory_min settings, IORM ignores the flashcachemin setting in the interdatabase IORM plan.