On 01/05/2012 12:53 PM, Tim Robinson wrote:
So with the original thread where with N=3 on 3 nodes. The developer
believed each node was getting a copy. When in fact 2 copies went to
a single node. So yes, there's redundancy and the "shock" value can
go away :) My apologies.
That said, I have no ability to assess how much data space that is
wasting, but it seems like potentially 1/3 - correct?
Another way to look at it, using the above noted case, is that I need
to double[1] the amount of hardware needed to achieve a single amount
of redundancy.
[1] not specifically, but effectively.
For the third time, no. Please read
http://wiki.basho.com/What-is-Riak%3F.html.
For 256 partitions, N=3:
Part Node
0 0
1 1
2 2
3 0
4 1
...
253 1
254 2
255 0
With n = 3, a key assigned to partition 0 will be stored on partition 0,
1, and 2.
Key Preflist Nodes
0 0,1,2 0,1,2
1 1,2,3 1,2,0
...
253 253,254,255 1,2,0
254 254,255,0 2,0,0 <-- overlap!
255 255,0,1 0,0,1 <-- overlap!
Only 1/128 of the data will reside on only two nodes. 127/128 of the
data will be distributed to three nodes. No data resides on only one
node. Data loss requires the simultaneous destruction of:
a. Any three nodes
b. Node 0 and 1
c. Node 0 and 2
This is true regardless of the number of nodes, so long as n does not
evenly divide N AND you are not using the workaround I linked to in my
previous post. If you do either of those things (use 4, 8, 16, or 32
nodes instead of 3, or use the n_val workaround), the distribution will
be even.
--Kyle
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