HP OpenVMS Systems

CI Bandwidth?

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The Question is:

 
What is the usable bandwidth of a CI bus?  Is
the following formula correct?
 
70 Mbit/s per channel, dual channel,
Max 140 Mbits/s if  Reads = Writes,
Normal I/O mix is 70% reads,
Max 91 Mbits/s = 11 MBytes/s Per CI
 
We are trying to determine if our two star
cluster needs three stars.
 
 

The Answer is :

 
  The CI bandwidth is seventy megabits per second dual path (140 megabits
  per second, aggregate), with recent CI controllers being able to operate
  on both paths in parallel.  The aggregate (theoretical) CI bandwidth
  is roughly 17.5 megabytes per second.
 
  What to Watch:
    o the kilobyte mapping rates,
    o the total message count and the average message size,
    o the average I/O queue depth,
    o the credit waits.
 
  Good indicators of general I/O subsystem saturation:
    o the I/O queue depth,
    o the credit waits.
 
  Also consider:
    o the bandwidth of the specific CI storage controllers,
    o the sustained I/O rates of the storage widgets,
    o the bandwidth of the particular CI adapters involved.
 
  Examples of recent (high-bandwidth capable) CI devices include the
  CIPCA and the HSJ-series controllers -- these can utilize more of
  the available CI bandwidth than earlier controllers.
 
  Tools such as MONITOR, DECamds, Capacity Planner, and similar are
  often used.  (The queue depth is visible via MONITOR DISK/ITEM=QUEUE)
 

  

     

     
answer written or last revised on ( 5-MAR-1999 )
     
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