Paging spaces placement and sizes

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The general recommendation is that the sum of the sizes of the paging spaces should be equal to at least twice the size of the real memory of the machine, up to a memory size of 256 MB (512 MB of paging space).

Note: For memories larger than 256 MB, the following is recommended:

total paging space = 512 MB + (memory size - 256 MB) * 1.25

However, with Deferred Page Space Allocation, this guideline may tie up more disk space than required. See Page space allocation for more information.

Ideally, there should be several paging spaces of roughly equal size, each on a different physical disk drive. If you decide to create additional paging spaces, create them on physical volumes that are more lightly loaded than the physical volume in rootvg. When allocating paging space blocks, the VMM allocates four blocks, in turn, from each of the active paging spaces that has space available. While the system is booting, only the primary paging space (hd6) is active. Consequently, all paging-space blocks allocated during boot are on the primary paging space. This means that the primary paging space should be somewhat larger than the secondary paging spaces. The secondary paging spaces should all be of the same size to ensure that the algorithm performed in turn can work effectively.

The lsps -a command gives a snapshot of the current utilization level of all the paging spaces on a system. You can also used the psdanger() subroutine to determine how closely paging-space utilization is approaching critical levels. As an example, the following program uses the psdanger() subroutine to provide a warning message when a threshold is exceeded:

/* psmonitor.c
  Monitors system for paging space low conditions. When the condition is
  detected, writes a message to stderr.
  Usage:    psmonitor [Interval [Count]]
  Default:  psmonitor 1 1000000
*/
#include <stdio.h>
#include <signal.h>
main(int argc,char **argv)
{
  int interval = 1;        /* seconds */
  int count = 1000000;     /* intervals */
  int current;             /* interval */
  int last;                /* check */
  int kill_offset;         /* returned by psdanger() */
  int danger_offset;       /* returned by psdanger() */


  /* are there any parameters at all? */
  if (argc > 1) {
    if ( (interval = atoi(argv[1])) < 1 ) {
      fprintf(stderr,"Usage: psmonitor [ interval [ count ] ]\n");
      exit(1);
    }
    if (argc > 2) {
      if ( (count = atoi( argv[2])) < 1 ) {
         fprintf(stderr,"Usage: psmonitor [ interval [ count ] ]\n");
         exit(1);
      }
    }
  }
  last = count -1;
  for(current = 0; current < count; current++) {
    kill_offset = psdanger(SIGKILL); /* check for out of paging space */
    if (kill_offset < 0)
      fprintf(stderr,
        "OUT OF PAGING SPACE! %d blocks beyond SIGKILL threshold.\n",
        kill_offset*(-1));
    else {
      danger_offset = psdanger(SIGDANGER); /* check for paging space low */
      if (danger_offset < 0) {
        fprintf(stderr,
          "WARNING: paging space low. %d blocks beyond SIGDANGER threshold.\n",
          danger_offset*(-1));
        fprintf(stderr,
          "                           %d blocks below SIGKILL threshold.\n",
          kill_offset);
      }
    }
      if (current < last)
        sleep(interval);
  }
}