Absolute consistency

Tim Robinson tim at blackstag.com
Thu Jan 5 15:12:32 EST 2012

Thank you for this info. I'm still somewhat confused.

Why would anyone ever want 2 copies on one physical PC? Correct me if I am wrong, but part of the sales pitch for Riak is that the cost of hardware is lessened by distributing your data across a cluster of less expensive machines as opposed to having it all one reside on an enormous server with very little redundancy.

The 2 copies of data on one physical PC provides no redundancy, but increases hardware costs quite a bit. 



-----Original Message-----
From: "Aphyr" <aphyr at aphyr.com>
Sent: Thursday, January 5, 2012 1:01pm
To: "Tim Robinson" <tim at blackstag.com>
Cc: "Runar Jordahl" <runar.jordahl at gmail.com>, riak-users at lists.basho.com
Subject: Re: Absolute consistency

On 01/05/2012 11:44 AM, Tim Robinson wrote:
> Ouch.
> 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.
> Tim

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balancing algo, so my explanation may be out of date.


Tim Robinson

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