Buffer Management › Planning Database Buffers › Choosing a Method for Storage Acquisition

Choosing a Method for Storage Acquisition

Choosing IDMS or OPSYS

The IDMS and OPSYS options on the BUFFER statements determine how CA IDMS/DB acquires storage for the buffer and the source of this storage:

• If you specify OPSYS storage, CA IDMS/DB issues one or more requests to the operating system for a contiguous block of storage. If the operating system supports extended addressing, the storage will be acquired above the 16-megabyte line.
• If you specify IDMS storage, CA IDMS/DB issues separate storage requests for each page in the buffer. The storage is acquired from IDMS-managed storage and will reside above the 16-megabyte line under the following conditions:
  • In local mode, if the operating system supports extended addressing
  • Under the central version, if an XA storage pool exists which supports system-type storage.

Advantages of Using OPSYS Storage

The OPSYS storage option offers an advantage to sites that define large buffers because of the way storage is acquired. For example, a buffer defined with an initial number of pages of 1000 will result in a single storage request for the entire 1000 pages if OPSYS is specified or 1000 storage requests if IDMS is specified. Another advantage is that the OPSYS storage is acquired outside the IDMS storage pool while IDMS storage is acquired from the IDMS storage pool. Therefore, the storage pool must be large enough to hold the buffer.

Insufficient Storage Under the Central Version

When initially allocating a buffer or when increasing the size of a buffer in response to a DCMT command, CA IDMS/DB may be unable to acquire all the necessary storage. If this occurs and the storage acquisition mode is OPSYS, CA IDMS/DB will attempt to acquire the storage from the IDMS storage pool. Whenever acquiring storage from the IDMS storage pool, if the necessary storage cannot be acquired or if the DC/UCF system is placed in a short-on-storage condition, the number of pages in the buffer is reduced by half until the necessary storage can be acquired without a short-on-storage condition.