Date: March 28, 2002
You may be wasting disk and I/O capacity if you have a large paging space. Starting with AIX 4.3.2, the rules for sizing page space changed**.
Previous to AIX 4.3.2, paging space needed to be large, typically 2-3 times the size of real memory. This is because page space was preallocated when a program started.....whether it used the page space or not.
Starting with AIX 4.3.2, paging space is not allocated until needed ("deferred paging"). In this environment, the system uses page space only if it runs out of real memory. If the memory is sized correctly, there is no paging and the page space can be small.
For example, I successfully ran a benchmark last summer on a p680 with 64 GB memory and a 32 MB page space. The application used less than 64 GB of real memory and it never touched the page space.
So my rule of thumb is to size page space proportional to the probability of paging. The lower the probability, the smaller the page space.
Here are a couple useful ways to determine how much memory and page space is being used by the system.
** The following URL lists the AIX functional enhancements by release: http://www-1.ibm.com/servers/aix/os/aixs2s.pdf
Who knows who else on Bruce's list might, too? We still haven't convinced them that they don't need to PSALLOC=early, but by a slight of hand, we've disabled their setting of the PSALLOC variable and the indicated processes still run correctly, we have oodles of paging space, and their code improved in performance by a factor of 3!!!!