ISAM pools are used as buffers for one or more NK-ISAM files. In addition to the buffers required for the individual PAM pages, they contain management data which describes, amongst other things, the files being buffered, the jobs connected to the pool, and the accesses to the individual buffers.
If, during ISAM processing, a block of an ISAM file is needed, the ISAM pool in which this file is buffered is searched first. If the block is not found in the pool, it has to be read from the disk. If all buffers of the pool are full, one of the blocks which is already in the buffer is overwritten; it is selected according to the following criteria: data blocks and overflow blocks are overwritten before index blocks; within a block type, the LRU (Least Recently Used) block is overwritten.
The handling of updated blocks can be defined for ISAM pools (and also for individual files) by using operands. Depending on the selected processing attributes, updated blocks are written back to disk either immediately or only when the buffer is needed to store another block. (For “immediate write”, see "WROUT function").
The following table provides an overview of macros and commands used to process tasklocal ISAM pools and to request information about these.
Macro | Command | Brief description |
|---|---|---|
ADDPLNK | ADD-ISAM-POOL-LINK | Assign a pool link name to an ISAM pool (task-specific). |
CREPOOL | CREATE-ISAM-POOL | Create a new ISAM pool or set up a link between the calling task and an existing ISAM pool. |
DELPOOL | DELETE-ISAM-POOL | Remove the link between the calling task and an ISAM pool. If the caller is the last or only task with a link to the ISAM pool, the ISAM pool itself is deleted as well. This macro can also be used to remove all links of the calling task to ISAM pools. |
REMPLNK | REMOVE-ISAM-POOL-LINK | Cancel the assignment of a pool link name to an ISAM pool. In this case (as in DELPOOL/DELETE-ISAM-POOL), a special parameter may be specified to remove all (task-specific) assignments of pool link names to ISAM pools by means of a single call. |
SHOPOOL | SHOW-ISAM-POOL-ATTRIBUTES | Return information on an ISAM pool to which the caller is connected at the time of the call. It is also possible to display information on all ISAM pools currently connected to the calling task. |
SHOPLNK | SHOW-ISAM-POOL-LINK | Show the assignments of pool link names to ISAM pools. |