5.2.3.6 Using Flash Cache Attributes

You can use flash cache attributes in the IORM plan to guarantee space allocation in Exadata Smart Flash Cache.

Using flash cache attributes, IORM can reserve space for critical databases while preventing less important or rogue entities from consuming the entire flash 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 flash cache resources. A hard limit means that the specified limit cannot be exceeded even when the memory 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.

Example 5-10 Configuring an Interdatabase Plan with Flash Cache Attributes

This example shows how to create an interdatabase plan with flash cache attributes.

In the example, the sales and test databases are allocated an amount of space in the flash cache by using the flashCacheSize parameter. But, the databases cannot exceed the specified allocation, even if the flash cache has free space.

The finance and dev databases use flashCacheMin for guaranteed minimum quotas. These databases can also exceed the specified flashCacheLimit size when there is free space in the flash cache.

ALTER IORMPLAN                                                           -
 dbplan=((name=sales, share=8, flashCacheSize=10G),                      -
         (name=finance, share=8, flashCacheLimit=10G, flashCacheMin=2G), -
         (name=dev, share=2, flashCacheLimit=4G, flashCacheMin=1G),      -
         (name=test, share=1, limit=10, flashCacheSize=1G))

Example 5-11 Configuring an Interdatabase Plan with Flash Cache Attributes

Starting with Oracle Exadata System Software release 19.3.0, you can specify both flashCacheMin and flashCacheSize for the same target.

ALTER IORMPLAN                                                  -
 dbplan=((name=sales, share=8, flashCacheMin=3G, flashCacheSize=10G),    -
         (name=finance, share=8, flashCacheLimit=10G, flashCacheMin=2G), -
         (name=dev, share=2, flashCacheLimit=4G, flashCacheMin=1G),      -
         (name=test, share=1, limit=10, flashCacheSize=1G))