5.1.3.6 About PMEM Cache Management in IORM Plans

Note:

This topic applies only to Oracle Exadata System Software releases before 23.1.0. Otherwise, see About XRMEM Cache Management in IORM Plans.

I/O Resource Management (IORM) can provide predictable performance by guaranteeing space in the persistent memory (PMEM) cache, which is available in Oracle Exadata Storage Server X8M and X9M.

With multiple databases and pluggable databases (PDBs) sharing the storage, PMEM 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 PMEM cache.

You can use the following attributes in the interdatabase plan to set limits for PMEM 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.

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

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

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

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

  However, if you set pmemCacheSize 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.

  pmemCacheSize is not a guaranteed reservation if the sum of the pmemCacheSize across all directives is more than the size of the PMEM cache. In this case, you can also specify pmemCacheMin 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 pmemCacheMin and pmemCacheSize settings for a database, then the PDB-specific memory_min quotas in the CDB plan represent fractions of the pmemCacheMin setting, and the PDB-specific memory_limit values represent fractions of the pmemCacheSize.

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