On Thu, Aug 28, 2014 at 9:52 PM, pragya jain <prag_2648 at yahoo.co.in> wrote: > I have some basic question about monitor and paxos relationship: > > As the documents says, Ceph monitor contains cluster map, if there is any > change in the state of the cluster, the change is updated in the cluster > map. monitor use paxos algorithm to create the consensus among monitors to > establish a quorum. > And when we talk about the Paxos algorithm, documents says that monitor > writes all changes to the Paxos instance and Paxos writes the changes to a > key/value store for strong consistency. > > #1: I am unable to understand what actually the Paxos algorithm do? all > changes in the cluster map are made by Paxos algorithm? how it create a > consensus among monitors Paxos is an algorithm for making decisions and/or safely replicating data in a distributed system. The Ceph monitor cluster uses it for all changes to any of its data. > My assumption is: cluster map is updated when OSD report monitor about any > changes, there is no role of Paxos in it. Paxos write changes made only for > the monitors. Please somebody elaborate at this point. Every change the monitors incorporate to any data structure, most definitely including the OSD map's changes based on reports from OSDs, is passed through paxos. > #2: why odd no. of monitors are recommended for production cluster, not even > no.? This is because of a trait of the Paxos' systems durability and uptime guarantees. -Greg Software Engineer #42 @ http://inktank.com | http://ceph.com
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