On Wed, 13 Feb 2019 13:50:17 +0100 Maciej S <inter...@swierki.com> wrote:
> Can you describe at least one situation when it could happen? > I see situations where data on two masters can diverge but I can't find the > one where data gets corrupted. Or maybe you think that some kind of > restoration is required in case of diverged data, but this is not my use > case (I can live with a loss of some data on one branch and recover it from > working master). With imagination and some "if", we can describe some scenario, but chaos is much more creative than me. But anyway, bellow is a situation: PostgreSQL doesn't do sanity check when starting as a standby and catching up with a primary. If your old primary crashed and catch up with the new one without some housecleaning first by a human (rebuilding it or using pg_rewind), it will be corrupted. Please, do not leave on a public mailing list dangerous assumptions like "fencing is like for additional precaution". It is not, in a lot a situation, PostgreSQL included. I know there is use cases where extreme-HA-failure-coverage is not required. Typically, implementing 80% of the job is enough or just make sure the service is up, no matter the data loss. In such case, maybe you can avoid the complexity of a "state of the art full HA stack with seat-belt helmet and parachute" and have something cheaper. As instance, Patroni is a very good alternative, but a PostgreSQL-only solution. At least, it has the elegance to use an external DCS for Quorum and Watchdog as fencing-of-the-poor-man and self-fencing solution. > śr., 13 lut 2019 o 13:10 Jehan-Guillaume de Rorthais <j...@dalibo.com> > napisał(a): > > > On Wed, 13 Feb 2019 13:02:30 +0100 > > Maciej S <inter...@swierki.com> wrote: > > > > > Thank you all for the answers. I can see your point, but anyway it seems > > > that fencing is like for additional precaution. > > > > It's not. > > > > > If my requirements allow some manual intervention in some cases (eg. > > > unknown resource state after failover), then I might go ahead without > > > fencing. At least until STONITH is not mandatory :) > > > > Well, then soon or later, we'll talk again about how to quickly restore > > your > > service and/or data. And the answer will be difficult to swallow. > > > > Good luck :) > > > > > pon., 11 lut 2019 o 17:54 Digimer <li...@alteeve.ca> napisał(a): > > > > > > > On 2019-02-11 6:34 a.m., Maciej S wrote: > > > > > I was wondering if anyone can give a plain answer if fencing is > > really > > > > > needed in case there are no shared resources being used (as far as I > > > > > define shared resource). > > > > > > > > > > We want to use PAF or other Postgres (with replicated data files on > > the > > > > > local drives) failover agent together with Corosync, Pacemaker and > > > > > virtual IP resource and I am wondering if there is a need for fencing > > > > > (which is very close bind to an infrastructure) if a Pacemaker is > > > > > already controlling resources state. I know that in failover case > > there > > > > > might be a need to add functionality to recover master that entered > > > > > dirty shutdown state (eg. in case of power outage), but I can't see > > any > > > > > case where fencing is really necessary. Am I wrong? > > > > > > > > > > I was looking for a strict answer but I couldn't find one... > > > > > > > > > > Regards, > > > > > Maciej > > > > > > > > Fencing is as required as a wearing a seat belt in a car. You can > > > > physically make things work, but the first time you're "in an > > accident", > > > > you're screwed. > > > > > > > > Think of it this way; > > > > > > > > If services can run in two or more places at the same time without > > > > coordination, you don't need a cluster, just run things everywhere. If > > > > you need coordination though, you need fencing. > > > > > > > > The role of fencing is to force a node that has entered into an unknown > > > > state and force it into a known state. In a system that requires > > > > coordination, often times fencing is the only way to ensure sane > > operation. > > > > > > > > Also, with pacemaker v2, fencing (stonith) became mandatory at a > > > > programmatic level. > > > > > > > > -- > > > > Digimer > > > > Papers and Projects: https://alteeve.com/w/ > > > > "I am, somehow, less interested in the weight and convolutions of > > > > Einstein’s brain than in the near certainty that people of equal talent > > > > have lived and died in cotton fields and sweatshops." - Stephen Jay > > Gould _______________________________________________ Users mailing list: Users@clusterlabs.org https://lists.clusterlabs.org/mailman/listinfo/users Project Home: http://www.clusterlabs.org Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf Bugs: http://bugs.clusterlabs.org
