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Compared to custom colocated clouds, you scale, code, and build your stack completely differently. I could not do half of what I do under any PaaS/SaaS.

I avoid disk at all costs (nearly unattainable amounts of RAM on PaaS/SaaS), if disks are hit they must be SSDs, treat everything immutably, concurrent/distributed computing, assume hardware is plentiful (192+GB ECC, 24+ of new xeon cores, etc). I scale completely differently than most. They really get you on RAM, I can build whole servers for what it might cost for a month of PaaS/SaaS.