I'm likely going to have to upgrade my hosting in order to accommodate the memory requirements of elasticsearch. That wasn't an issue with sphinx. The difference in hosting costs for me (alone) over the course of a year would likely pay for sphinx development costs.
For around 80 USD per month you can get a server with 32 GB RAM. RAM is really cheap now. I am happy that there is an official solution for big board search available directly from the developers. And I got very good support from them when I installed it. It is really good, I'd rely on this solution.
Funny that everyone wants to debate what the precise cost of the ram upgrades would be, rather than addressing the fact that 1) it requires more memory and 2) there are direct and indirect costs associated with obtaining those extra resources, so therefore 3) elasticsearch costs more money to support than sphinx.
Sphinx cost me several hundred of euros for buying, installing and maintaining the forum solution on vbulletin. Not mentioning the time I invested in that technology. I understand that each community is different, so for you sphinx can be more affordable. I am very happy with elasticsearch.
I have allocated a half, 1 or 2 GB or RAM to elasticsearch, I don't remember. It was according to the advices for a 1-2 Million Post Board. My server has 8 GB RAM and around one million posts, so I have much more RAM than I need. I went from vb 4.1 to xf. To be honest, for this size of a forum I don't think one needs elasticsearch, I just installed it as it may have very minor advantages to the build in xf search.
Those memory requirements are based upon a very high userload all hammering the search.
It also assumes that you are using the default mapping, which, given advanced configuration, you could in theory half those memory requirements.
However advanced configuration of ES was not included in those threads as many people venturing into ES for the first time will a) not need it and b) the larger (5m+) boards will most likely already be aware of the advanaced mapping.
1&1 isn't what I'd consider a 'quality' hosting company. Also, based on the bargain price (and the lack of documentation to the contrary), that's likely consumer-grade (rather than enterprise-grade) hardware. It's also more money per month than I'm currently paying.... although the system stats are substantially better.
I don't think any of it is relevant to the current discussion, but I wouldn't want someone reading this to think that was a 'good buy' or anything like that. They're likely to be disappointed when a drive goes bad in their dedi box, and the hosting company takes it offline for half a day while they unrack, service, and rerack the machine.
I would be interested in some more configuration info on ES, though. Anything that can be done to lessen its memory requirements is time well spent.