What are the advantages of using CA Datacom Presspack given that DASD is inexpensive?
We typically recommend the use of a compression package when at least 40-50 of the average data footprint could be "saved". The reason for this is that every read must use the decompression routine to expand the row and every add/update must use the compression routine to shrink the row. The cost of decompress/compress is the CPU cycles it takes to process the row. The cost is same if the process shrinks the row by 1% or 50%. So if we stopped there it would be just a comparison of the cost of the extra DASD versus the cost of the extra CPU cyles.
Because the Datacom Multi-User (MUF) is a highly buffered system that works best when it can service data reads from existing buffers, the use of a compression package can greatly increase the number of data rows that can be stored in a data block and/or data buffer. Since, a compressed table will most likely have more rows per buffer the number of times a request can be serviced by a buffer read versus a DASD IO is improved. Every time we read from an buffer there is a CPU saving because there is no IO issued, and there is no need to dump an existing buffer to load the new block.
So now we must balance the extra DASD cost against the (extra CPU to compress/decompress minus the CPU saved by the buffer).
The reasoning follows that the more data compression that can be done, the more rows that fit per block/buffer = the more CPU savings. There any other considerations to also consider such as it takes less MRDF memory to cover a compressed data area, MUF has several built in processes to keep heavily accessed "compressed rows" in memory as long as possible.
So the answer is even though DASD is an inexpensive cost, having Datacom data areas with significant compression can out perform the same data area uncompressed.
With that said, one major differentiation that CA Datacom Presspack offers over any non-CA compression package. ISVs (like CA) are under contract with IBM to use zIIP processors. That contract requires us to agree that we will only "zIIP" code that we own. Therefore, the Presspack compression exit can be invoked in MUF and executed fully on the MUF WLM SRB (zIIP enabled) but we are not permitted to zIIP enable non-CA code that is issued under the compression exit.