aphyr at aphyr.com
Thu Jan 5 15:01:18 EST 2012
On 01/05/2012 11:44 AM, Tim Robinson wrote:
> I'm shocked that is not considered a major bug. At minimum that kind of stuff should be front and center in their wiki/docs. Here I am thinking n 2 on a 3 node cluster means I'm covered when in fact I am not. It's the whole reason I gave Riak consideration.
I think you may have this backwards. N=3 and 2 nodes would mean one node
has 1 copy, and 1 node has 2 copies, of any given piece. For n=2 and 3
nodes, there should be no overlap.
The other thing to consider is that for certain combinations of
partition number P and node number N, distributing partitions mod N can
result in overlaps at the edge of the ring. This means zero to n
preflists can overlap on some nodes. That means n=3 can, *with the wrong
choice of N and P*, result in minimum 2 machines having copies of any
given key, assuming P > N.
There are also failure modes to consider. I haven't read the new key
balancing algo, so my explanation may be out of date.
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