The value of Performance Testing is to warranty,
that the system is fit for use
(has the appropriate level of availability, security, and performance).
Scability Testing buys us confirmation of predictions about what will happen before it happens,
buying the lead time to do the right thing when additional capacity is needed.
Load testing provides measurements for Capacity Managers
to anticipate the true capacity of IT resources: whether it can really support the
peak workloads
anticipated.
Stress (Overload) testing
identifies the predicted point of failure
where servers fail to handle loads.
The biggest actionable concern is time needed to recover from overload.
Most web hosts today control overload by issuing "Service Unavailable".
The existing capacity of the system is ideally defined by
the usable capacity at a point of load where users notice slow response time is noticeable.
This is obtained by conducting Speed (performance) testing
The difference between the current load (the actual demand)
and usable capacity from load testing is the real reserve capacity
— the amount of "head room" for growth or the ability to handle variation in demand.
The work of capacity management is finding a balance between the unused expense
of having too much idle capacity against the
risk of reputation-damaging problems from not enough capacity.
Upgrades to capacity can be smoother if there is what
the ITIL methdology calls a common Forward Schedule of Changes (FSC).
Lead time include time for planning and testing,
which can be shortened by a more agile approach.
The approximate date when usable capacity will be reached can be calculated
by dividing into the usable reserve capacity the rate capacity usaage is growing
(per day).
Subtracing the lead time from that date yields the the Trigger Point
when upgrade work should begin.
Subtracting the amount of capacity growth during the lead time
yields the threshold of workload which should trigger an upgrade.
Thus, this chart gives actionable meaning to production monitoring.
Capacity Projection Management with Performance Testing
Performance and Capacity Management...
