I just want to monitor my sites only. As for spec, I got around 30K unique vistors per day. I don't know what server is needed for such traffic.
Well, you woud probably want something close to your server that has your website - that is for sure.
I'd safely say a 512mb RAM VPS would do - even less maybe. One think you'll want to consider is drive space as you'll need somewhere to store that data.
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Piwik is nice because for high traffic sites you can set a cron to run to avoid overloading your Piwik server with constant updates and do them in batches. While real-time updates are ideal it also requires more resources for your Piwik server so you can balance cost with performance by utilizing the cron.
Since this involved php and mysql, I'd recommend you to go at least with 1GB Xen VPS.
This will ensure the stability and efficiency of you monitoring. Be sure you looking for server grade CPU to ensure the performance.
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