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Date:      Thu, 19 Apr 2007 12:28:16 +0200
From:      Oliver Brandmueller <ob@e-Gitt.NET>
To:        freebsd-stable@freebsd.org
Subject:   Re: iscsi and geom mirror - stupid idea or not ?
Message-ID:  <20070419102815.GG95707@e-Gitt.NET>
In-Reply-To: <E1HeTFQ-0009lC-DQ@dilbert.ticketswitch.com>
References:  <E1HeTFQ-0009lC-DQ@dilbert.ticketswitch.com>

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Hi,

On Thu, Apr 19, 2007 at 10:48:16AM +0100, Pete French wrote:
> what would happen if I made a machine which contained a mirrored
> geom pair consiting of one local driive and one drive accessed via iscsi =
on
> a remote machine ? would this work ?
>=20
> what I am considering is two such machines, geographicly distinct. one is
> a 'master' and boots off the mirrored drive, the other is a slave and
> has a separate boot drive which just rngs FreeBSD to make the drive inside
> it into an iscsi target for the first machine. The idea here is if the fi=
rst
> machine is catastrphicly killed (like building falls down on it or
> something) then the second one can be rebooted from the internal drive, a=
nd
> will hence become the first one. It's basically a way of making a standby
> machine in case of disaster.
>=20
> I havent really looked at iSCSI until recently, and this is just one of
> the ideas I came up with looking at the possibilities.

You could also go and use ggate for that. And seems to get more and more=20
common to work like that, although probably most setups I heard of=20
probably don't have a long distance link between them.

There are a few things you should consider: First, you have to make=20
absolutely sure, that for example the mirrored disk is not attached if=20
after a crash the original master comes back and the slave took over. If=20
this happens you're likely to damage something really bad.

Second is, something like this gives you mirrored data with practically=20
no gap to the original disk. The price you have to pay: This does not=20
help you against logical errors (a filesystem damage will be replicated=20
just fine...). A setup like this does not serve as a backup.

Third is, you'll have to fsck everything, so this defines your minimum=20
service outage. I'm not sure, if I'd trust background fsck here, also bg=20
fsck is a big performance penalty, which might or might not be a=20
problem for your setup.


A replication (like rsync, ssync or similar) sure has the drawback of=20
the replication gap for the data. Also you cannot just take over the IP=20
of the NFS server, but have to remount everything. But you have fsck=20
time, lower chance damage due to logical error and the nice effect, that=20
you could do your backups from the replicated data, not affecting your=20
live system. But have to deal with lost data from probably several hours=20
or how to replicate changed data after recovery.


- Olli


--=20
| Oliver Brandmueller | Offenbacher Str. 1  | Germany       D-14197 Berlin |
| Fon +49-172-3130856 | Fax +49-172-3145027 | WWW:   http://the.addict.de/ |
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