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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/
> _______________________________________________
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