HP OpenVMS Systems

Volume Shadowing Merge Delay Factor?

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The Question is:

 
The ECO ALPSHAD09_062 introduced the ability to control the rate of a volume
 shadow merge by defining the system logical name SHAD$MERGE_DELAY_FACTOR.
If I change the value of this logical name during a merge, how long does it
 take for the merge software to notice the change, and change the merge rate?
 

The Answer is :

 
  Faster I/O subsystems including the HSZ50 and HSZ70 controllers were
  taking longer to perform full shadowset merges than were some older
  and slower I/O subsystems.  Accordingly, changes were made to volume
  shadowing to permit the system manager to adjust certain thresholds.
 
  Two new logical names permit the system manager to vary the merge
  multiplication factor used for each virtual unit, and to vary it on
  a per-node basis.
 
  The logical names must be defined in the system table, and should be
  defined on each cluster member.
 
  The valid range for a threshold is from 100 to 1000, or from 100 to
  10000 on later versions.   The default value is 200, and is used when
  no value is set or when a value outside the supported range is set.
 
  The current value is displayed at the start of a shadow set merge, in
  the %SHADOW_SERVER-I-SSRVINIMRG message.
 
  Excessively large values can cause application performance degradation
  due to system load during shadowset merges.  The value used is a tradeoff
  between the shadowset merge speed and site-specific application
  performance requirements.
 
  THe logical names are retranslated every one thousand I/Os, permitting
  potential adjustments during merges.
 
  The logical names are:
 
    SHAD$MERGE_DELAY_FACTOR_DSAnnnn
 
  Where nnnn is the shadowset volume unit number to apply the specified
  delay factor.  The higher the delay factor, the faster the merge and
  the higher the system overhead.  If the consistency across the shadowset
  members is less time-critical or if the shadowset merge is interfering
  with the application I/O to the shadowset members, a lower delay factor
  can be selected.
 
  The system-wide default value can be established via the logical name:
 
    SHAD$MERGE_DELAY_FACTOR
 
  After each one thousand I/Os, the delay factor logical name is translated.
  The default delay factor value is 200, and the maximum value is 1000 or
  (in later versions) 10000.  With a minimal delay factor value set, there
  exists the potential for multiple hours between re-translations of the
  logical names.
 
  The shadowset full copy I/O transfer size can be established via the
  following logical name:
 
    DEFINE/SYSTEM SHAD$COPY_BUFFER_SIZE 31
 
  This establishes the minimum buffer size value of 15872 bytes (512 bytes
  per block times 31 blocks).
 

  

     

     
answer written or last revised on ( 15-MAR-2001 )
     
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