Jack, The recommended ensemble configured size takes into consideration that you might have a node failure. You can still run with two while you replace the third, so it's sort of like RAID-5.
If you run with four configured nodes, you're still running with RAID-5-like failure survival characteristics. Two configured can't be the minimum because the minimum requires an odd number of machines to break ties, so your minimum is three when you configure for four. Configuring for five nodes lets you survive two failures because you now have a minimum of three Michael Della Bitta ------------------------------------------------ Appinions 18 East 41st Street, 2nd Floor New York, NY 10017-6271 www.appinions.com Where Influence Isn’t a Game On Thu, Dec 6, 2012 at 5:25 PM, Jack Krupansky <j...@basetechnology.com>wrote: > Slightly more recent link: > http://zookeeper.apache.org/**doc/r3.4.5/zookeeperAdmin.html<http://zookeeper.apache.org/doc/r3.4.5/zookeeperAdmin.html> > > -- Jack Krupansky > > -----Original Message----- From: Jack Krupansky > Sent: Thursday, December 06, 2012 5:21 PM > > To: solr-user@lucene.apache.org > Subject: Re: Minimum HA Setup with SolrCloud > > Rewind. > > If 1 is the minimum, what is the 3 "minimum" all about? > > The zk web page does say "Three ZooKeeper servers is the minimum > recommended > size for an ensemble, and we also recommend that they run on separate > machines" - but it does say "recommended". > > But back to the original question - it sounds as if 4 nodes would the > recommended minimum number of nodes if you want to tolerate one machine > going down and maintaining that 3 zookeeper recommended minimum ensemble. > > From the zookeeper web page: > > "For reliable ZooKeeper service, you should deploy ZooKeeper in a cluster > known as an ensemble. As long as a majority of the ensemble are up, the > service will be available. Because Zookeeper requires a majority, it is > best > to use an odd number of machines. For example, with four machines ZooKeeper > can only handle the failure of a single machine; if two machines fail, the > remaining two machines do not constitute a majority. However, with five > machines ZooKeeper can handle the failure of two machines." > > See: > http://zookeeper.apache.org/**doc/r3.1.2/zookeeperAdmin.html<http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html> > > -- Jack Krupansky > > -----Original Message----- From: Otis Gospodnetic > Sent: Thursday, December 06, 2012 4:39 PM > To: solr-user@lucene.apache.org > Subject: Re: Minimum HA Setup with SolrCloud > > 1 is the minimum :) > 2 makes no sense. > 3 must be the most common number in the zoo. > > Otis > -- > Performance Monitoring - > http://sematext.com/spm/index.**html<http://sematext.com/spm/index.html> > Search Analytics - > http://sematext.com/search-**analytics/index.html<http://sematext.com/search-analytics/index.html> > > > > > On Thu, Dec 6, 2012 at 9:46 AM, Jack Krupansky > <j...@basetechnology.com>**wrote: > > "but if those same machines are running zookeeper, you need 3." >> >> And one of those 3 can go down? I thought 3 was the minimum number of >> zookeepers. >> >> -- Jack Krupansky >> >> -----Original Message----- From: Mark Miller >> Sent: Thursday, December 06, 2012 9:30 AM >> To: solr-user@lucene.apache.org >> Subject: Re: Minimum HA Setup with SolrCloud >> >> >> It depends on if you are running embedded zk or an external zk ensemble. >> >> One leader and a replica is all you need for Solr to allow on machine to >> go down - but if those same machines are running zookeeper, you need 3. >> >> You could also run zookeeper on one external machine and then it would be >> fine if you lost one solr node - but if you the one external zk node went >> down you would lose the ability to do updates until it was brought back >> up. >> >> - Mark >> >> On Dec 6, 2012, at 2:46 AM, Thomas Heigl <tho...@umschalt.com> wrote: >> >> Hey all, >> >>> >>> I'm in the process of migrating a single Solr 4.0 instace to a SolrCloud >>> setup for availability reasons. >>> >>> After studying the wiki page for SolrCloud I'm not sure what the absolute >>> minimum setup is that would allow for one machine to go down. >>> >>> Would it be enough to have one shard with one leader and one replica? >>> Could >>> this setup tolerate one of the instances going down? Or do I need three >>> instance because Zookeeper needs a quorum of instances? >>> >>> Cheers, >>> >>> Thomas >>> >>> >> >>
