Specifying IOBUF buffer values

If you want to maximize real storage usage, choose a buffer size (bufsize) that fits efficiently onto a page. The following calculations can be made to determine an optimum value for your environment. When selecting a value, you need to know the average PIU size to have a starting point for bufsize.

Note: For type 2 local SNA devices, the actual data size is 16 bytes less than BUFSIZE.

To calculate how many buffers fit on a page (no encryption specified):

Determine the bufsize value.
Add 91 bytes to the specified bufsize value.
Round the sum up to a doubleword boundary.
Divide 4096 by the value.
The quotient indicates the number of buffers that can fit on 1 page (for the given bufsize value).
The remainder indicates the number of bytes that are not used on each page. However, these unused bytes are taken into consideration for the xpanlim value that you specify. Keep this number as low as possible.

If the encryption start option is specified (ENCRYPTN = YES, 24, or 31), add 98 to the bufsize value instead of 91.

Table 1 reflects the IOBUF formula and can be used when coding bufsize. For greatest efficiency, choose the high number in the range, because the higher number has a smaller remainder.

Note: The minimum bufsize used by VTAM® is 256. If a bufsize of less than 256 is specified, VTAM overrides the value with a value of 256 and displays no message indicating this has occurred. D NET,BFRUSE,BUFFER=IO will, however, show the resulting change.

Table 1. IOBUF buffer size and number of buffers per page
Number of buffers per page	Range of buffer sizes for MVS (with 4k paging)	Range of buffer sizes for MVS using data encryption facility (with 4k paging)
1	1958–3992	1951–3992
2	1270–1957	1263–1950
3	934–1269	927–1262
4	726–933	719–926
5	590–725	583–718
6	494–589	487–582
7	422–493	415–486
8	358–421	351–414
9	318–357	311–350
10	278–317	270–310
11	256–277	256–270