Difference between Performance and Load testing

1. Performance Testing
In white box testing; testers, developers, system administrators and DBAs work together in order to instrument the application code and the database queries (via specialized profilers for example), and the hardware/operating system of the server(s) running the application and the database (via monitoring tools such as vmstat, iostat, top or Windows PerfMon). All these activities belong to performance testing.
The goal of performance testing is not to find bugs, but to eliminate bottlenecks and establish a baseline for future regression testing.
These bottlenecks can exist at multiple levels:
a. at the application level, developers can use profilers to spot inefficiencies in their code (for example poor search algorithms) 
b. at the database level, developers and DBAs can use database-specific profilers and query optimizers 
c. at the operating system level, system engineers can use utilities such as top, vmstat, iostat (on Unix-type systems) and PerfMon (on Windows) to monitor hardware resources such as CPU, memory, swap, disk I/O; specialized kernel monitoring software can also be used 
d. at the network level, network engineers can use packet sniffers such as tcpdump, network protocol analyzers such as ethereal, and various utilities such as netstat, MRTG, ntop, mii-tool. 
From a testing point of view, the activities described above all take a white-box approach, where the system is inspected and monitored "from the inside out" and from a variety of angles. 
2. Load Testing
Are you actively profiling your application code and/or monitoring the server(s) running your application? If the answer is yes, then you're engaged in performance testing. If the answer is no, then what you're doing is load testing. 
In the testing literature, the term "load testing" is usually defined as the process of exercising the system under test by feeding it the largest tasks it can operate with. Load testing is sometimes called volume testing, or longevity/endurance testing.
Examples of volume testing
a. testing a word processor by editing a very large document 
b. testing a printer by sending it a very large job 
c. testing a mail server with thousands of users mailboxes 
d. a specific case of volume testing is zero-volume testing, where the system is fed empty tasks
Examples of longevity/endurance testing
a. testing a client-server application by running the client in a loop against the server over an extended period of time
Goals of load testing
a. expose bugs that do not surface in cursory testing, such as memory management bugs, memory leaks, buffer overflows, etc. 
b. ensure that the application meets the performance baseline established during performance testing. This is done by running regression tests against the application at a specified maximum load. 
Although performance testing and load testing can seem similar, their goals are different. On one hand, performance testing uses load testing techniques and tools for measurement and benchmarking purposes and uses various load levels. On the other hand, load testing operates at a predefined load level, usually the highest load that the system can accept while still functioning properly. Note that load testing does not aim to break the system by overwhelming it, but instead tries to keep the system constantly humming like a well-oiled machine.
3. Stress Testing
If you want to actually discover bugs and to see if your application fails and recovers gracefully, you need to do stress testing. Blast your Web server with double the number of users for example. Unplug network cables randomly (or shut down/restart switch ports via SNMP). Take out a disk from a RAID array.
The main purpose behind this madness is to make sure that the system fails and recovers gracefully.

Here are some ways in which stress can be applied to the system: 
a. double the baseline number for concurrent users/HTTP connections 
b. randomly shut down and restart ports on the network switches/routers that connect the servers (via SNMP commands for example) 
c. take the database offline, then restart it 
d. rebuild a RAID array while the system is running 
e. run processes that consume resources (CPU, memory, disk, network) on the web and database servers 
If chair is designed for 100 kg weight, and my weight is 70 kg then that testing is called as normal testing. If my weight is 100 kg then that testing is called as load testing. If my wt is 120 kg then that testing called as stress testing.
Difference between Performance and Load testing
a. Performance test: To determine or validate speed, scalability, and/or stability.
b. Load test: To verify application behavior under normal and peak load conditions
c. Stress test: To determine or validate an application’s behavior when it is pushed beyond normal or peak load conditions.
d. Capacity test: To determine how many users and/or transactions a given system will support and still meet performance goals.

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