5.2.3.10 Using PMEM Cache Attributes

Note:

This topic applies only to Oracle Exadata System Software releases before 23.1.0. Otherwise, see Using XRMEM Cache Attributes.

You can use PMEM cache attributes in the IORM plan to guarantee space allocation in the persistent memory (PMEM) cache.

Using PMEM cache attributes, IORM can reserve space for critical databases while preventing less important or rogue entities from consuming the entire PMEM cache. These attributes can only be specified in an interdatabase plan, and are configured using the CellCLI utility.

You can use the following attributes to set limits for PMEM cache resources. A hard limit means the database cannot exceed its quota even when the memory 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.

Example 5-13 Configuring an Interdatabase Plan with PMEM Cache Attributes

This example shows how to create an interdatabase plan with pmem cache attributes. In the example, the sales and test databases are guaranteed an amount of space in the PMEM cache by using the pmemCacheSize parameter. But, the databases cannot exceed the specified allocation, even if the PMEM cache has free space.

The finc and dev databases use pmemCacheMin for guaranteed minimum quotas. These databases can also exceed the specified pmemCacheLimit size when there is free space in the PMEM cache.

The example plan also contains various flash cache attributes.

ALTER IORMPLAN dbplan=                                            -
((name=sales, share=8, pmemCacheSize= 2G, flashCacheSize=10G), -
(name=finc, share=8, pmemCacheMin= 1G, pmemCacheLimit= 2G, flashCacheLimit=10G, flashCacheMin=2G), -
(name=dev, share=2, pmemCacheMin= 500M, pmemCacheLimit= 1G, flashCacheLimit=4G, flashCacheMin=1G), -
(name=test, share=1, limit=10, pmemCacheSize= 200M))