Each free list group is associated with one or more instances at startup. Using free list groups also partitions data. Blocks allocated to one instance, freed by another instance, are no longer available to the first instance. With multiple free list groups, the free list structure is delinked from the segment header, thereby reducing contention for the segment header. The free list options must be specified for the whole cluster, rather than for individual tables.
Clusters are an optional method of storing data in groups of tables that have columns in common.
Related rows of two or more tables in a cluster are physically stored together within the database to improve access time. Some hash clusters can also use multiple free lists and free list groups, if they are created with a user-defined key for the hashing function, and the key is partitioned by instance. Use these options in the same manner as described for tables. When multiple instances concurrently insert rows into a table having an index, contention for index blocks decreases performance unless index values can be separated by instance. For example, consider a database where space is pre-allocated to the free list groups in a table and users are distributed across instances such that the data is well partitioned, with the result that minimal pinging of data blocks occurs.