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From Rick Branson <rbran...@datastax.com>
Subject Re: RFC: Cassandra Virtual Nodes
Date Tue, 20 Mar 2012 14:50:25 GMT
> > I like this idea. It feels like a good 80/20 solution -- 80% of the
> > benefits, 20% of the effort. More like 5% of the effort. I can't
> > even enumerate all the places full vnode support would change, but an
> > "active token range" concept would be relatively limited in scope.
> 
> 
> It only addresses 1 of Sam's original 5 points, so I wouldn't call it
> an "80% solution".
> 
To support a form of DF, I think some tweaking of the replica placement could achieve this
effect quite well. We could introduce a variable into replica placement, which I'm going to
incorrectly call DF for the purposes of illustration. The key range for a node would be sub-divided
by DF (1 by default) and this would be used to further distribution replica selection based
on this "sub-partition". 

Currently, the offset formula works out to be something like this:

offset = replica

For RandomPartitioner, DF placement might look something like:

offset = replica + (token % DF)

Now, I realize replica selection is actually much more complicated than this, but these formulas
are for illustration purposes.

Modifying replica placement & the partitioners to support this seems straightforward,
but I'm unsure of what's required to get it working for ring management operations. On the
surface, it does seem like this could be added without any kind of difficult migration support.


Thoughts?



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