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Customer use case: analysis of a completely overloaded system

We started an analysis on customer side because of pending decisions how to extend the available capacities.
these are the results after the first look (1h) - the most interesting thing here is that the system is not looking to bad on the SCSI level - but when one looks deeper ...

The Customer has performance issues since a longer time and there is a need to clarify the root causes before spending money.

There are several ideas starting from newer Nodes (it is a 2145-8G4 node) or higher performance storage classes

The result might be - get rid of the old N6060 storage with the performance issues as planned and invest in medium performance capacities
because there is no real need for highest performance classes.
The Cache on the 2145-8G4 nodes is small - we have to go deeper to find whether it is a technical need to invest in newer SVC nodes.

Customer Session on 4. April 2003Click on pictures to enlarge

The IO Heat Analysis Overlay in the Treemap shows
that the customer does not use the performance capabilities
of his backed storage.


This shows - that enough performance capability is
available. (IN THEORY)

In Praxis - if the systems in the storage backend cannot
deliver the performance they are built for than all these
calculations are for nothing.

The result is then, that the performance of storage
backend systems is not enough beause of several
reasons.

The processor of the system is too slow. It doesn't help
when we build in hundreds of disks for performance when
the processor is not able to serve all theses IOS.

Something is speeding down the performance like
a WAFL file system which is too full to perform striping.
(which might be the case here)

The coloring is based on calculations we did for the Arrays.

  • blue - does only use less than 20% of the
    performance capabilities
  • Green between 20% and 80%
  • Yellow between 80% and 100%
  • Red more than 100%

 

 

The CPU Load and RW Cache is in a normal range.


The Global Write Cache max is much too high -
it should normally be from 0% up to less than 80%.

In this situation the write Cache max is mainly mote
than 75% which is much too high.


This is a normal loaded System

  • (red) CPU << 70%
  • (blue) RW Cache SVC = 78
  • (blue) RW Cache Storwize = 72
  • (green) Global W - Cache max < 80%
  • (dark green) (not shown here)
    Global W - Cache min often 0

This is one of the managed disk groups from
the customer.

The Max Write Cache fullness is nearly always
more than 80%.

 

It looks like the SVC cannot get rid of the cache
content because of restricted performance in
the storage back end.


  • (green) MDG W cache max - sometimes 100%
    for small timeframes. Normally should be
    at 80% or less. Long times on 100% is overload
    especially when MDG W Cache min is over 80%.
    More than 100% only single peaks. In this case
    watch out for min value - has to be <80%

  • (dark green) (not shown here)
    MDG W Cache min - should often be 0 sometimes
    for smaller periods 80% seldom more than 80%
    90% and more is alarm signal

The SAN does not show any problems.
In the time period of cache overflow no heavy
transfer activity can be measured on the SAN
ports and no Buffer Credit Wait % problems occor.

 

  • (Yellow) Buffer Credit Wait % - always measure this
    on the single port (never on aggregate).
    less than 10% is OK as long as there are no other
    performance complaintsin this moment.

  • All other codes from the debug section have to be 0

Another Sign of congestion is that the response time
of the SAP volumes go in parallel with their IOP demand.

 

More IOPS lead automatically to a higher response time.




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