could a tax rate based on hashrate be implemented?... at least for a short time
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The problem isn't really the pool hashrates, it's the net hashrate.
My pool, has averaged between 25MH/s and 15 MH/s most of the day (currently at the lower end, had a few long rounds the last hour or two) and Valid Error has been sat in the 50's most of the day (some wild swings for both, but otherwise pretty constant). Our pools are remaining pretty stable, but the net hashrate (maybe not totally accurate all the time) has been swinging from under 100MH/s to over 200MH/s.
It the net rate that's making Valid Error go over 50% so regularly, which is not the fault of the pool. In the last hour, the combined hashrates of
http://noble.pool-mining.com and
http://noble.valid-error.com/ have been around 70/75MH/s, but the total Network Hashrate has been 180 at a high, and 86 (now) as a low. Unless something is reporting wrong in the network hashrates, it means our two pools are anything from 40%, to 85%+ of the total for the coin - and that's with nothing really changing in the two pools themselves.
If we can manage the massive swings in total hashrates, then it's possible to manage the pools, but pool operators can't balance hashrates very well in this situation. If I were to up my fee's to 5%, it may take several hours for miners to notice - and leave - by which time, the total net hashrate may have exploded thus negating the need for a single pool to reduce users, hurting the pool and not having much effect on the coin at all.
I don't know how you'd manage the total network hashrate, but I think we all agree we need to see it either stabalise, or grow at a steadier pace - rather than swing wildly 150% in a matter of a couple hours...
All that being said, my pool will be retaining 0% fees until at least Monday (or if we go above 50%, unlikely) to try and retain the total net hash rate we currently have. I'm looking at putting in higher block rewards too, but given the 0% fee, I'm not sure I personally have enough in my wallet to make this that attractive.