Thank you, Mike Sage and Greg. Completely different than everything I had heard or read. Clears it all up. :)
Gracias, -bo On Thu, Jun 20, 2013 at 11:15 AM, Gregory Farnum <g...@inktank.com> wrote: > On Thursday, June 20, 2013, Bo wrote: > > > > Howdy! > > > > Loving working with ceph; learning a lot. :) > > > > I am curious about the quorum process because I seem to get conflicting > information from "experts". Those that I report to need a clear answer from > me which I am currently unable to give. > > > > Ceph needs an odd number of monitors in any given cluster (3, 5, 7) to > avoid split-brain syndrome. So what happens whenever I have 3 monitors, 1 > dies, and I have 2 left? > > > > The information regarding this situation that I have gathered over the > past few months all falls within these three categories: > > A) commonly "stated"--nothing is said. period. > > B) rarely stated--this is a bad situation (possibly split-brain). > > C) rarely stated--each monitor has a "rank", so the highest ranking > monitor is the boss, thus quorum. > > > > Does anyone know with absolute certainty what ceph's quorum logic will > do with an even number of (specifically 2) monitors left? > > > > You may say, "well, take down one of your monitors", to which I > respectfully state that my testing is not an authoritative answer on what > ceph is designed to do and what it does in production. My testing cannot > cover the vast majority of cases covered by the hundreds/thousands who have > had a monitor die. > > > > Thank you for your time and brain juice, > > -bo > > > This is often misunderstood, but the answers to your questions are > pretty simple. :) > > There is no risk of split brain in Ceph (so, not in the monitor either). > The mantra to use an odd number of monitors is *not* a system > requirement; it is a deployment recommendation. This is due to how the > cluster avoids split brain — using a Paxos variant in which a strict > majority of the monitors need to agree on everything. Using one > monitor, you can make forward progress if it's running; using two > monitors, you can afford for neither of them to die (because then you > only have 50% up); using three monitors you can lose one; using four > you can lose one; using five you can lose two; etc. So using an even > number of monitors increases your odds of failure without increasing > your survivability (in availability terms) of failure over the > previous odd number. > -Greg > Software Engineer #42 @ http://inktank.com | http://ceph.com > -- "But God demonstrates His own love toward us, in that while we were yet sinners, Christ died for us. Much more then, having now been justified by His blood, we shall be saved from the wrath of God through Him." Romans 5:8-9 *All *have sinned, broken God's law, and deserve eternal torment. Jesus Christ, the Son of God, died for the sins of those that will believe, purchasing our salvation, and defeated death so that we all may spend eternity in heaven. Do you desire freedom from hell and be with God in His love for eternity? "If you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved." Romans 10:9
_______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
