HP OpenVMS Systems

Introduction to Paging and Swapping?

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The Question is:

 
I have a VAX 7640 running OpenVMS V6.2 with 1GB of memory. The system is
 configured with 3 PAGEFILE.SYS of approximately 1 million blocks each. All
 three are on non-system disks. There is no SWAPFILE configured or installed.
My system administrator says that if required, the system will use the
 available PAGEFILE for process swapping. My question(s) are :
1) Is that true? 2) Is it advisable? 3) What if "Reservable" PAGEFILE is a
 negative number? 4) Doesn't a "minimum" VMS boot require a PAGEFILE.SYS on the
 system disk?
 
 

The Answer is :

 
  OpenVMS can and will swap to the PAGEFILE if necessary, and if the
  PAGEFILE has sufficient space, and if this behaviour has not been
  disabled via the dynamic system parameter NOPGFLSWP.
 
  Swapping to the PAGEFILE is generally not an issue, as most OpenVMS
  systems -- save for those that are severely memory-constrained -- do
  not swap.  If the processes are swapped out, they first get their
  working sets trimmed back, thus relatively few pages need be swapped.
 
  For reasons of performance, pagefiles should generally be oversize,
  and they can (should) be scattered across disk spindles, based on
  the system, paging, and disk spindle I/O loads.
 
  A minimally booted OpenVMS system with 1 GB of memory will operate
  for a considerable interval before the system is likely to stall.
  (OpenVMS does not support processes that are started up before the
  PAGEFILE(s) are connected, and that run for long intervals.  Cases
  that are known to have caused problems include bootstrapping with
  STARTUP_P1 set to MIN (and with no pagefiles connected), followed
  by logging in and performing many image activations -- it best to
  mount the disks, connect the pagefiles, and SPAWN to perform the
  necessary commands.)
 
  Swapping is generally only common when it is more efficient to
  free up system memory by tossing out large processes in their
  entirety.  See the system parameter SWPOUTPGCNT for details on
  how to control this.  When swapping occurs "naturally" on a
  memory-constrained OpenVMS system, system performance is generally
  such that swapping to the SWAPFILE or to the PAGEFILE is not a
  central performance issue -- the system is better served through
  the acquisition of more physical memory or the reduction of peak
  physical memory requirements.
 
  For details on the negative number in the reservable pages, please
  see the FAQ section "MGMT17. Why do I have negative number in the
  pagefile reservable pages?"
 

  

     

     
answer written or last revised on ( 23-MAY-2000 )
     
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