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How do you do Load Testing and Capacity Planning for Web Sites

I was wondering if there is a way to actually determine how many users my VPS can handle until it will run out of Memory. I have a small VPS with 3 assigned CPU's and 4 GB RAM.

This specific machine runs on WHM/cPanel , CentOS 5 x 64 and NGINX. I host several very small (private) and one medium sized Magento store ( ~ 2500 products + Memcached enabled). This store has several thousand page-views a day.

In recent days however I have noticed that this specific store throws out an error log of exhausted RAM. I reckon this happens when too many users are on the store at the same time. Performing an upgrade to more RAM and additional CPU's would generally not a problem, but before I do that I would like to know if there is an actual way how many users or simultaneous visits my store can handle so I can choose the correct upgrade for me.

I am certainly not a pro therefore I would very much appreciate some expert advise.

marked as duplicate by John Gardeniers, Michael Hampton, Magellan, Ladadadada, Scott Pack Oct 22 '12 at 18:05

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I would say it is hard to guess what you really need, but an approach would be cloning the shop onto a local vm and benchmarking e.g. via apache benchmark while monitoring the ram/cpu usage of the vm. Since you know how many users access your shop, you can take an average value or a guessed peak value for a concurrent user visit and benchmark that too.

So, for e.g 1'000 requests, 10 concurrent, you'd do ab -c 10 -n 1'000 <url>. The result would be a report on the times the requests took etc. Combined with watching your cpu/ram, this should give you some base to guess what upgrade your VPS needs.

  • Thank you very much for the insight and suggestions - I will have a look into it! – TheDave Oct 10 '12 at 5:31

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