HP OpenVMS Systems

Calculating optimal ACP_WINDOW value?

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The Question is:

 
How do you calculate the optimal value for either the ACP_WINDOW Sysgen
Parameter or the set volume/window=xx parameter for disk drives.  My
understanding is the followng formula
 
acp_window =
      (largest file size of all disks)/65535
 

The Answer is :

 
  The effect of insufficient ACP_WINDOW is a high window turn rate.
 
  The cost of a high window turn rate is often overrated in the opinion
  of the OpenVMS Wizard, mostly because the windows are 'backfilled' from
  file headers, which in turn are often cached.
 
  The cause of an insufficient ACP_WINDOW setting is often excessive disk
  fragmentation.  This is often avoidable (at no cost!) by using adequate
  initial file allocation and significant (default) file extent sizes,
  as well as an occasional pass of BACKUP and restoration when needed.
  (Needed both due to current fragmentation levels and due to the relative
  age of the last good disk BACKUP.)  Consider SET RMS/EXTENSION [/SYSTEM]
  and SET VOLUME/EXTENSION, as well as available FDL parameters.
 
  If the OpenVMS Wizard recalls correctly, the (older) 65535 limitation
  was rendered obsolete starting with the OpenVMS V6.0 release.
 
  Having written all that, the default of 7 pointers is low.  If you are
  indeed monitoring significant window turning, then consider doubling
  the value for some busy disk, and monitor further.  For example, issue
  the command SET VOLUME/WINDOW=20
 
  If you choose to significantly increase the window size for a large number
  of open files, you will want to evaluate (and potentially increase) the
  values for the process BYTLM quota and the system non-paged pool size.
 

  

     

     
answer written or last revised on ( 1-SEP-1999 )
     
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