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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[-- Attachment #1 --] 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 ? > > 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 first > machine is catastrphicly killed (like building falls down on it or > something) then the second one can be rebooted from the internal drive, and > will hence become the first one. It's basically a way of making a standby > machine in case of disaster. > > 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 common to work like that, although probably most setups I heard of probably don't have a long distance link between them. There are a few things you should consider: First, you have to make absolutely sure, that for example the mirrored disk is not attached if after a crash the original master comes back and the slave took over. If this happens you're likely to damage something really bad. Second is, something like this gives you mirrored data with practically no gap to the original disk. The price you have to pay: This does not help you against logical errors (a filesystem damage will be replicated 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 service outage. I'm not sure, if I'd trust background fsck here, also bg fsck is a big performance penalty, which might or might not be a problem for your setup. A replication (like rsync, ssync or similar) sure has the drawback of the replication gap for the data. Also you cannot just take over the IP of the NFS server, but have to remount everything. But you have fsck time, lower chance damage due to logical error and the nice effect, that you could do your backups from the replicated data, not affecting your live system. But have to deal with lost data from probably several hours or how to replicate changed data after recovery. - Olli -- | Oliver Brandmueller | Offenbacher Str. 1 | Germany D-14197 Berlin | | Fon +49-172-3130856 | Fax +49-172-3145027 | WWW: http://the.addict.de/ | | Ich bin das Internet. Sowahr ich Gott helfe. | | Eine gewerbliche Nutzung aller enthaltenen Adressen ist nicht gestattet! | [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.3 (FreeBSD) iD8DBQFGJ0Q/iqtMdzjafykRAjCrAJ0SG8hfxsXWi2bIN8yQ7Ei8kHrswwCcDV0l RyBBFKqykr8bX6WlNgxAhIU= =Mucs -----END PGP SIGNATURE-----
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