PAGING SPACE LOW messages will be generated when 512 pages remain free in the pool of free pages. Processes will be terminated when only 128 pages of free memory are left.
This is a very common message in a Unix Administrator life. When it comes , in most of circumtances , administrator can not any thing ;sometimes he can not even login to system;sometimes he can login but can not execute any commands.He will get the message "Can not Fork" which is indicative of very low paging space condition on AIX.
The only way to recover from this situation is to restart the system and then monitor paging space utilization for some longer time and then analyze whether you need to optimize paging space on AIX.
As a rule of thumb , all paging spaces on AIX should be roughly of same size and split across multiple physical volumes. However hd6 can be slightly bigger than others.
AIX determines which users have the most page space allocated to them and selects those processes for termination. All real memory allocated to a process will have a backing store of equal size (for every page of real memory allocated, there will be a page of disk space from paging space allocated)
Its recommended that the first paging space (/dev/hd6) be larger, since this one is brought on sooner than the rest, resulting in being more full than the others.
Thrashing should not be confused with the problem of low paging space. Low paging space is the condition in which the amount of paging space is insufficient. Thrashing is the condition in which the amount of RAM is insufficient. Low paging space involves the consumption of disk space; thrashing involves the consumption of RAM and disk I/O.
Example:lsps -a (Display attributes of all paging spaces)or lsps -s which give a summary view
LV hd6 is the default paging space. If more than one paging LV is defined, hd6 will always have a higher percentage of utilization since this is the first paging LV turned on at boot time. Once all the other paging LVs have been swapped on, paging is allocated on a round robin basis - four pages (pagesize is 4k ) at a time.
80% (MAXPERM) of real memory is to be used by persistent storage. Persistent storage is information that is not paged to the page space (/dev/hd6) but rather is paged to the physical volume where that file resides.
aix no longer guarantees pages in real memory with a backing store. this is why the requirement for double real memory is no longer true, has been the case since AIX 5L.
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