Date: Fri, 6 Nov 2015 00:24:20 -0600 From: Josh Paetzel <josh@tcbug.org> To: "Michael W. Lucas" <mwlucas@michaelwlucas.com> Cc: fs@freebsd.org Subject: Re: hast exec vs devd for handling CARP events Message-ID: <64BC4405-50B7-424F-9CB6-00A6475C4204@tcbug.org> In-Reply-To: <20151103173526.GA17299@mail.michaelwlucas.com> References: <20151103173526.GA17299@mail.michaelwlucas.com>
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Both hast and carp have an amazing propensity to go split brain. This has de= vastating affects. Hundreds of years ago sailing vessels could determine their latitude by shoo= ting the sun with an astrolabe and knowing the precise time. (You can also c= alculate the time if you know your latitude and have an astrolabe) The wisdom of that time was go to sea with one clock or three. If you have o= ne clock you have no choice but to believe it. If you have three you believe= the two that agree. If you have two clocks and they disagree you are in a w= orld of hurt. Such is it with computers too. You really need three systems to do HA. The b= est you can do with two systems is require a positive ACK from both nodes be= fore one node will take over, otherwise you are forced to go passive-passive= and require administrative action to proceed. Any other strategy will event= ually hit a situation where it goes split brain. If you are in a directly connected universe you can use SPC-3 SCSI reservati= ons to lock the disks and prevent the nodes from destroying the storage. In a= hast universe you really need quorum. Thanks, Josh Paetzel > On Nov 3, 2015, at 11:35 AM, Michael W. Lucas <mwlucas@michaelwlucas.com> w= rote: >=20 > Hi, >=20 > There's lots of recipes out there for HAST failover based on devd. >=20 > HAST also has the ability to run scripts on events, with the exec > function in hast.conf. >=20 > It *seems* it make more sense to have HAST mount filesystems and start > processes when it claims the master role for a resource, as opposed to > triggering that mount via a devd event and waiting for HAST to perform > the switch. >=20 > Thanks for any insight. I'm researching the "specialty filesystems" > book, and want to give the best advice. >=20 > =3D=3Dml >=20 > --=20 > Michael W. Lucas - mwlucas@michaelwlucas.com, Twitter @mwlauthor=20 > http://www.MichaelWLucas.com/, http://blather.MichaelWLucas.com/ > _______________________________________________ > freebsd-fs@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org"
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