I run Redis with PHP-FPM/Nginx/MySQL all on the same 512. Redis acts as a cache and session store in this case. Another case runs Nginx/NodeJS/Redis where redis is the main datastore, again on a 512.
In other words you really don't need to stick it on a separate box, redis is single threaded so you won't chew up your CPU, disk IO is negligible, the only problems you will have is if you start to max out your memory. Putting redis on a separate box does make upgrading easier since you don't have to take everything else offline while you resize your Linode.
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