Hi Ted and Jared
Thank you for your replies. Sorry in advance if my questions are too
absurd.
I know that, currently, with two servers the cluster would have a
higher failure probability (since the ensemble started with three and it needs
a two nodes majority, if any one fails there is no longer a majority and the
cluster hangs right?).
The purpose of the quorum disk would be to function has a tie breaker
in even node deployments (namely two), by representing an external, storage
only, resource where cluster state would be maintained. I know there are some
paxos-based deployments that use this technique, although I'm not sufficiently
into the internals of zookeeper to assert whether it would work.
A question about an alternative... would it be possible to change the
majority function (dynamically) in two node deployments?
Regarding zookeeper-107, that is very interesting as we were
considering using zookeeper in an "elastic" setting. Is there progress on this
issue? can I help?
Regards
David
On Dec 6, 2010, at 5:02 PM, Ted Dunning wrote:
David,
For your second point, take a look at
https://issues.apache.org/jira/browse/ZOOKEEPER-107. Unfortunately,
Zookeeper does not support this feature yet.
~Jared
> You can definitely do this, but your reliability will be different from what
> might be expected. The probability of the cluster hanging due to a failure
> is higher than for a single machine. The probability of data loss will be
> lower than for a single machine. Similarly, any maintenance that requires
> one of the two machines to be down will cause your cluster to hang.
>
> Can you clarify what you mean by 2 server + 1 quorum disk?
>
> Btw... one way around the downtime during maintenance is to reconfigure the
> cluster on the fly to use a single server
> during the maintenance window. You will still have a short window of freeze
> because you need to take down the
> server that is exiting the cluster first before bouncing the other server
> with the new config. If your maintenance period is less than a few minutes,
> this isn't usually worthwhile.
>
> On Mon, Dec 6, 2010 at 7:43 AM, David Alves <[email protected]> wrote:
>
>> 1- Feasibility of a two node cluster:
>> Q- I know zookeeper runs over (a approx. version of) paxos
>> and tolerates f failures in 2f+1 nodes but would it be possible to use in a
>> 2 server + 1 quorum disk deployment (much like windows clusters do)? The
>> objective would be to use ZK both for distributed (processing nodes >2) and
>> single processing node (2 nodes active-passive) highly available
>> deployments.
>>
