Transcript Capacity planning

Capacity planning for web sites
Promoting a web site
• Thoughts on increasing web site traffic
but…
• Two possible scenarios…
Scenario 1: sudden traffic increase
• Web site causes its own surge – new
marketing
• New link to another popular site
• Site mentioned by popular index – slashdot
effect
• Site hosts a web application used by other
applications
Scenario 2: cumulative increase
• Occurs as site becomes more widely known
• Introduction of new technologies e.g. XML
increases load
• Network effect – increase in connections to
a site from other sites
Result
Web server
inundated with
requests
Resulting
processes cause
memory
overload
Web server
slows then
crashes
Performance issues
• Speed is first problem
• Components of web systems –possible
bottlenecks
• Content – one web page = many files
• Increased web services- increased no of hits
• Web traffic – randomness of accesses
• User’s perception of performance
Solution – capacity planning
• Estimates space, hardware, software,
infrastructure needed over future period
• Takes into account projected growth in site
usage
• “Just in time” – add the capacity just before
it is needed – no unused resources
Answers the questions
Future needs?
“What if..” scenarios
Will performance be scalable?
e.g. is bandwidth adequate?
e.g. can system handle load spikes?
What monitoring tools are available?
How can more capacity be provided?
Why is capacity planning important?
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Ensure positive user experience
Maximise productivity
Avoid financial surprises
Maintain stability and availability
Arrange for emergency capacity
Assess performance of new
applications/architectures
Tittel (2002) “Why plan web capacity?”
“Because capacity problems can’t be
solved instantaneously…”
(Menasce & Almeida (2001))
Planning capacity
• Some definitions
• Clarifying requirements
• Planning activities
– Monitoring
– Predicting
– Estimating
• Action and review
www.simac.be/solutions/ management/3.htm, accessed 27.4.05
Some definitions
• Relating to server
– Service level
– Service time
– Waiting time
•Relating to web site
–Hits per second
–Errors per second
–Time-out rate
• Relating to network
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Latency
Throughput
Bandwidth
Utilization
Quality of service
Clarifying requirements
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What is the purpose of the site? How available should it be?
How tolerant are the users?
Have you analysed log files?
How many HTTP operations per unit time expected?
What is the average transfer size going to be?
Will you provide any streaming media?
Will the web server spawn additional processes?
What other processes need to be run on the web server or over the
network?
• What sort of scalability do you need?
• What is your budget? Can you force suppliers to compete?
• Future traffic and usage growth – spare capacity levels?
Killelea (2002)
Planning activities
Understand environment
Cost
model
Workload characteristics
Workload
model
Workload prediction
Monitor performance
Performance
model
Performance prediction
Cost/performance analysis
Plans
Adapted from Menasce
& Almeida (2001)
Workload prediction
• Differentiate between types of workload
• Characteristics, e.g.
– “Burstyness”
– Large transfers
• Analysis
– Capacity trending
– Analytical modelling
Measuring and predicting
performance
• Which are the critical web site IT resources
• Average & “under-load” response times
• Web server performance benchmarks and
monitoring tools
• Scenario data sets
• Load testing and modelling
Plan for…
• How much bandwidth?
– Hits/second x average size of hit in bits = bits/second
• How fast a server?
– Load balancing
– Load clustering
– Mirror servers
• How much memory?
– Memory-using components of server:
– Operating system, web server configuration, web content, serverside programs
And…
• Establish upgrade process for resources
• Scalability – capacity interdependencies
between infrastructure components
• Monitor actual traffic growth and resource
usage
• Revise plan when any component changes
(kit, traffic or market)
Ways of providing capacity/improving
performance
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Reconfigure network
Rewrite critical applications
Reduce content load
Proxy, caching and mirror servers
Test using your performance model
What influence can you have as a web
administrator?
– Can’t directly influence Internet
– May not be able to influence resource decisions
• Write down capacity requirement
• Monitor performance, keep records
• Alter server configuration – performance
and content
• Plan for future scalability