On 13/06/2013, at 1:57 AM, Digimer <li...@alteeve.ca> wrote: > On 06/12/2013 03:06 AM, Michael Schwartzkopff wrote: >> Am Mittwoch, 12. Juni 2013, 09:42:13 schrieb Andrew Beekhof: >> > On 12/06/2013, at 4:48 AM, Michael Schwartzkopff >> <mi...@clusterbau.com> wrote: >> > > Am Dienstag, 11. Juni 2013, 22:33:32 schrieb Andrey Groshev: >> > > > Hi, >> > > > I want to make Postgres cluster. >> > > > As far as I understand, for the proper functioning of the cluster >> must >> > > > use a quorum (ie, at least three nodes). >> > > No. Two nodes are enough. See: no-quorum-policy="ignore". >> > > > But if the databases are large - it is >> > > > too expensive. >> > > > One master, two slave, and in addition backup. >> > > > So I'm trying to make the schema with one master, one slave, and one >> > > > node >> > > > only for a quorum. >> > > One master and one slave should be enough. >> > But three nodes is better. >> Definitely yes. But if he has financial limitations (see above) it is >> better to invest in good fencing than in a third node. > > I build exclusively two-node clusters, and the biggest draw-back is the > possibility of a "fence loop". That is, without quorum and with a network > error, a node can come up on it's own, fail to contact it's peer and fence > it. When the fenced node boots, it comes up, fails to contact it's peer, and > fences it. Wash, rinse, repeat. > > To prevent this, I recommend *not* letting your cluster stack start on boot. > This does mean that you need to manually start the node(s) on boot, but I > have found this to not be an issue. The only times this is needed is when I > am cold-booting the cluster or after a node has failed and been fenced. > > In the former case, it is usually at the end of scheduled maintenance so I am > there to do it anyway. In the latter case, I don't want a node to rejoin the > cluster until I've been able to look into the failure first anyway. > > I also recommend, as a matter of course but doubly so in two-node clusters, > to have redundant fence methods. My personal favourite combination is IPMI as > the first fence method with switched PDUs as the backup fence method. Run > IPMI through your primary fence and the PDUs through your backup switch and > you have good fencing, even if a switch or link has failed.
Its certainly possible to build a decent 2-node cluster, but there are several non-obvious steps that are required - preventing fencing loops being one. For this reason I cannot recommend them for newcomers, because they are also the most likely group to be unaware of the caveats. _______________________________________________ Pacemaker mailing list: Pacemaker@oss.clusterlabs.org http://oss.clusterlabs.org/mailman/listinfo/pacemaker Project Home: http://www.clusterlabs.org Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf Bugs: http://bugs.clusterlabs.org
