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Date:      Thu, 08 Apr 1999 12:07:26 +0900
From:      "Daniel C. Sobral" <dcs@newsguy.com>
To:        Greg Lehey <grog@lemis.com>
Cc:        Ollivier Robert <roberto@keltia.freenix.fr>, FreeBSD Hackers <hackers@FreeBSD.ORG>
Subject:   Re: Separate boot partition?
Message-ID:  <370C1D6E.C447CB11@newsguy.com>
References:  <19990407085435.M2142@lemis.com> <19990407080113.A4122@keltia.freenix.fr> <19990407155835.M2142@lemis.com> <19990408000955.D8314@keltia.freenix.fr> <19990408095949.G2142@lemis.com>

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Greg Lehey wrote:
> 
> A lot of the differences are conceptual.  What are volume groups?
> They correspond in position to plexes, but I believe they're different
> I think in IBM VGs (or whatever they're called there) contain the
> replicated data for part of a volume.  A plex contains one copy of the
> data for the entire volume.

A volume group is a collection of physical volumes. You can have it
replicating a single physical volume two or three times (ie, two or
three hd with replicated data). It cannot replicate data that spans
more than one physical volume (ie, you can't have four physical
volumes replicated 2-2). This replication feature is a minor
feature, though. The main use of volume groups is in a collection of
non-replicated physical volumes, over which you'll be allocating
logical volumes. You can add and subtract physical volumes to/from a
volume group.

A volume group contains information about the logical volumes it's
logical volumes and their mappings over the volume group (you can
remap a logical volume -- moving it out of a disk you want to
replace, for instance). It also has the "boot sectors". This
information is replicated in each physical volume.

It also has a, err, feature where you can specify a volume group
automatically shuts down once the number of active physical volumes
(and inactive one being one that crashed) gets below 50%.

I wish my AIX books were not on the other side of the world... :-(

--
Daniel C. Sobral			(8-DCS)
dcs@newsguy.com
dcs@freebsd.org

	"nothing better than the ability to perform cunning linguistics"




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