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From Andor Molnar <an...@cloudera.com>
Subject Re: Configuring SolrCloud with Redundancy on Two Physical Frames
Date Wed, 02 May 2018 12:23:26 GMT
Hi Adam,

Zk uses quorum to get agreement between voting servers (leader and
followers). Quorum is calculated by majority of servers: N / 2 + 1, where N
is the number of servers. When you only have 2 Zk servers running, the
quorum equals 2 which means you need to have both servers up and running
for ZooKeeper to work. In this case you only have a live backup of the data
basically, but you don't have fault tolerance.

If you would like to have fault tolerance, you need at least a 3rd server
"physically" separated with approx the same horsepower as the others. If
you have a weak machine and it's voted for leader, you'll lose write
performance, if it's voted for follower, you'll lose read performance.

Running 2 physical machines as one, redundant hypervisor might be a good
option. You can run 3 VMs on top of that and if one of your servers goes
down, the other one will be able to run all 3 VMs which keeps ZK
operational.

Hope that helps a little bit on Zk side.

Regards,
Andor







On Tue, May 1, 2018 at 9:52 PM, Shawn Heisey <apache@elyograg.org> wrote:

> On 5/1/2018 8:55 AM, Adam Blank wrote:
> > I would like to have a high-availability/redundant installation of
> > Zookeeper running in my production environment. The problem is that I
> only
> > have 2 physical frames available, so that rules out configuring a
> Zookeeper
> > cluster/ensemble since I'd only have redundancy if the frame with the
> > minority of servers goes down. What is the best practice in this
> situation?
> > Is it possible to have a separate standalone install running on each
> frame
> > connected to the same set of SOLR nodes or to use one server as primary
> and
> > one as backup?
>
> I'm more familiar with the Solr side of the equation than the ZK side.
> That said, I do know if you want bulletproof fault tolerance for ZK, you
> must have at least three separate physical installations.
>
> For redundancy of Solr itself in SolrCloud, you only need two hosts, but
> for ZK, you need three.  The minimal fault tolerant SolrCloud setup
> would run both Solr and ZK on two servers and a third ZK by itself.
> That third server would not need much horsepower.  I've heard of people
> using a laptop or a low-end desktop for the third server.
>
> For best results, you would also want redundancy beyond the servers
> themselves -- power and network for each server ideally would be
> separate hardware.  Some even go as far as separate racks and/or
> geographically diverse datacenters.
>
> Thanks,
> Shawn
>
>

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