If an ISAM file is opened without the user having assigned it to a specific ISAM pool in the FILE/FCB macro (POOLLNK operand) or in the ADD-FILE-LINK command (POOL-LINK operand), DMS uses a system standard ISAM pool for processing the file: in the case of an OPEN with SHARUPD=NO, a task-local standard ISAM pool is allocated, and in the case of an OPEN with SHARUPD=YES a cross-task standard ISAM pool.
A task-local ISAM pool is characterized by the fact that it can be used by only one task. When an ISAM file is opened for the first time with SHARUPD=NO, the DMS creates the ISAM pool $TASK01. This pool is also used for further ISAM file processing in the same task, provided there is sufficient space for each file. Otherwise the pool $TASK02 is created when the next ISAM file is opened, and so on.
Up to 16 ISAM pools ($TASKn, 01 <= n <= 16) can be created for each job. Systems support defines the size of the pools using the ISAM parameter LCLDFPS.
A cross-task ISAM pool can be used by all tasks in the system. The first time an ISAM file is opened with SHARUPD=YES, DMS creates such a pool and as a rule only uses it for this file. If sufficient space is available, a separate pool is created for each further file which is opened with SHARUPD=YES.
In contrast to task-local pools, cross-task pools do not have a name. They are implicitly linked to the file they contain. Systems support defines their size using the ISAM parameter GLBPS and their maximum number indirectly using the ISAM parameter MAXDSBN. This parameter, which can be modified during ongoing operation using the MODIFY-ISAM-CACHING command, specifies how many data spaces DMS may use to accommodate cross-task ISAM pools. The maximum number of pools which may be generated is calculated from the fixed maximum size of a data space (2 GB) and the size of the pool.
One job may simultaneously process several ISAM files which can be allocated to different standard pools.
When an ISAM file which was processed using a standard pool is closed, DMS initially checks whether the job is still linked to this pool via other files. If this is not the case, it releases the resources reserved by this pool.
The pages of a cross-task ISAM pool in a data space which contain user data are not output in memory dumps. Only the management data of such pools which is required for diagnostics appears in any memory dumps which may need to be created.