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From "Buttler, David" <buttl...@llnl.gov>
Subject RE: Queries on Zookeeper failure and RegionServer restartup
Date Wed, 21 Sep 2011 20:34:10 GMT
Hmm... Interesting perspective.  From my point of view, if you add a node to an odd-numbered
set, the reliability of the system will decrease.  This is easiest to see between 1 and 2
nodes.  For 2 nodes, if either node fails the whole system dies.  P(A*B) < P(A) -- adding
the second node can only decrease your reliability.  By induction you can see that reliability
always decreases by adding a node to an odd numbered set.

So, no, it is not a *requirement* that zookeeper have an odd number of nodes.  It is just
self-defeating for that not to be the deployment plan.  Although, if you did happen to have
a 200 node zookeeper system, it is not going to be that much less resilient than a 199 node
system.  But that would be silly -- at that scale you have other issues that are much more
pressing.

Dave

-----Original Message-----
From: Joe Pallas [mailto:joe.pallas@gmail.com] On Behalf Of Joe Pallas
Sent: Wednesday, September 21, 2011 9:34 AM
To: user@hbase.apache.org
Subject: Re: Queries on Zookeeper failure and RegionServer restartup


On Sep 20, 2011, at 8:37 AM, Buttler, David wrote:

> Wait, you do realize that you have to have a majority of zookeeper nodes alive for zookeeper
to work, right?  That means that you get lower reliability with two nodes than one node: if
either node goes down, zookeeper will give up.  This also implies that you need to have an
odd number of nodes in your zookeeper ensemble.

I keep seeing this last part repeated and I believe it is a misunderstanding.  There is no
requirement for an odd number of nodes, although adding a node to an odd-numbered set will
not increase reliability.  "For this reason, ZooKeeper deployments are usually made up of
an odd number of machines." <http://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_CrossMachineRequirements>

joe


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