HP OpenVMS Systems

Shadowing I/O Load?

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The Question is:

 
Over the past 10 years or so I have repeatedly heard the following statement:
 
during a full shadow copy operation, the shadow server process limits the
 number of IO's to 20 per second for that particular DSA device.
 
Meaning, appart from the IO's that are done by the shadow server process in
 order to complete the shadow coppying operation, the DSA device is limited to
 20 IO's / second for users, programs etc.
 
Is this true?
 
Best Regards.. Joe
 

The Answer is :

 
  The Shadow_Server process has NEVER -- since the product's inception
  in the V5.4-1 release -- limited the rate at which is issues $QIOs to
  the SHDRIVER during a "full shadow copy operation".
 
  Having said that, the terms merge and copy are regularly confused,
  though each refers to a distinct and different operation.
 
  The Shadow_Server process does use an algorithm to limit the rate at
  which it issues $QIOs to the SHDRIVER during a full merge operation.
  That algorithm can be influenced by using system logical names:
 
  For all virtual units connected on a node:
    SHAD$MERGE_DELAY_FACTOR
 
  For a specific virtual unit in a node:
    SHAD$MERGE_DELAY_FACTOR_DSAnnnn
 
  Details of this logical name are in the ECO kit release notes for the
  shadowing-related kits for V6.2 thorough V7.2-1H1.
 
  As for the reference to 20 I/Os per second, while a very early version
  of the SHDRIVER did involve throttling read requests served by the
  master member when the UCB$L_QLEN of the members reached 20.  That has
  long since been replaced with another throttling algorithm.
 
 
  In any case, the throttling limits the number of QIO's issued by the
  shadowing code, so that it will not compete too much with user QIO's.
  No throttling is applied to user QIO's, although they will see reduced
  performance because of the competing I/O performed by the merge.
 

  

     

     
answer written or last revised on ( 29-SEP-2000 )
     
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