(CC'ing ceph-users) On 07/28/2014 12:34 PM, Marc wrote: > Hi, > >> This said, if out of 3 monitors you have 2 monitors down, your cluster >> will cease functioning (no admin commands, no writes or reads served). > > This is not entirely true. (At least) RBDs will continue being fully > functional even if the mon quorum is lost. This only applies to RBDs > that are already mounted (qemu) at the time of quorum loss though. > > Meaning: (K)VMs running off of Ceph will remain fully functional even if > the mon quorum is lost (assuming you havent lost too many OSDs at the > same time). True. Clients will maintain the connections they have to OSDs for about 15 minutes or so, at which point timeouts will go off and all work will be halted. New clients won't be able to do this though, as they have to grab maps from the monitors prior to connecting to OSDs, and the monitor will not serve those requests if quorum is not in place. -Joao > > On 28/07/2014 12:22, Joao Eduardo Luis wrote: >> On 07/28/2014 08:49 AM, Christian Balzer wrote: >>> >>> Hello, >>> >>> On Sun, 27 Jul 2014 18:20:43 -0400 Robert Fantini wrote: >>> >>>> Hello Christian, >>>> >>>> Let me supply more info and answer some questions. >>>> >>>> * Our main concern is high availability, not speed. >>>> Our storage requirements are not huge. >>>> However we want good keyboard response 99.99% of the time. We >>>> mostly do >>>> data entry and reporting. 20-25 users doing mostly order , invoice >>>> processing and email. >>>> >>>> * DRBD has been very reliable , but I am the SPOF . Meaning that when >>>> split brain occurs [ every 18-24 months ] it is me or no one who knows >>>> what to do. Try to explain how to deal with split brain in advance.... >>>> For the future ceph looks like it will be easier to maintain. >>>> >>> The DRBD people would of course tell you to configure things in a way >>> that >>> a split brain can't happen. ^o^ >>> >>> Note that given the right circumstances (too many OSDs down, MONs down) >>> Ceph can wind up in a similar state. >> >> >> I am not sure what you mean by ceph winding up in a similar state. If >> you mean regarding 'split brain' in the usual sense of the term, it does >> not occur in Ceph. If it does, you have surely found a bug and you >> should let us know with lots of CAPS. >> >> What you can incur though if you have too many monitors down is cluster >> downtime. The monitors will ensure you need a strict majority of >> monitors up in order to operate the cluster, and will not serve requests >> if said majority is not in place. The monitors will only serve requests >> when there's a formed 'quorum', and a quorum is only formed by (N/2)+1 >> monitors, N being the total number of monitors in the cluster (via the >> monitor map -- monmap). >> >> This said, if out of 3 monitors you have 2 monitors down, your cluster >> will cease functioning (no admin commands, no writes or reads served). >> As there is no configuration in which you can have two strict >> majorities, thus no two partitions of the cluster are able to function >> at the same time, you do not incur in split brain. >> >> If you are a creative admin however, you may be able to enforce split >> brain by modifying monmaps. In the end you'd obviously end up with two >> distinct monitor clusters, but if you so happened to not inform the >> clients about this there's a fair chance that it would cause havoc with >> unforeseen effects. Then again, this would be the operator's fault, not >> Ceph itself -- especially because rewriting monitor maps is not trivial >> enough for someone to mistakenly do something like this. >> >> -Joao >> >> > -- Joao Eduardo Luis Software Engineer | http://inktank.com | http://ceph.com
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