This parameter can be used to request the buffer space for the datasets usually VSAM datasets. when the datasets are accessed they are accessed only in terms of blocks for example when accessing a record from a dataset, the record is read from a block and when all the records from the block are read then the next block is accessed and so on. so as to place these blocks we require buffers which are specified using this parameter. here BUFNI will hold the Index part and BUFND will hold the data part of the VSAM file.
Could you please tell why this AMP parameter is not always mandatory whenever we access a VSAM file? Because I sometimes see it is used with some VSAM files and sometimes not.
The AMP= parameter is only used for VSAM files and mainly affects performance.
When VSAM files are defined via the DEFINE CLUSTER statement using IDCAMS, certain parameters describe the physical format of the file, while others define default performance charateristics of the file depending on how it will be most often used (e.g. Sequentially read, Randomly Read, Update and Delete activity, etc.).
The AMP (Access Method Parameters) option allows these performance options to be overridden at runtime in the event that the file is going to be processed in a different manner than normal.
In the case of BUFNI(Index Buffers) and BUFND(Data Buffers), these parameters affect the amount of buffer space reserved for frequently used data in memory, to avoid the repeated I/O's to reread data that has already been read. These buffers can greatly aid performance when a file is read sequentially, using a concept known as "Anticipated Read", which causes not only the requested record to be read, but also the "anticipated" next records in the file. When the next record is requested, it is already in a buffer(memory) and the system does not need to perform an I/O to get it.
In an online(CICS) environment, these parameters become very important, as different CICS applications may process the same VSAM file in different manners.
If you are processing the file in the manner intended by whoever created the original definition, then the AMP overrides should not be needed.
What is AMP parameter and its use e.g. AMP='BUFNI=10,BUFND=10' means what?