Date: Mon, 25 Jan 1999 08:56:35 -0600 (CST) From: "Jasper O'Malley" <jooji@webnology.com> To: Greg Lehey <grog@lemis.com> Cc: John Saunders <john.saunders@nlc.net.au>, freebsd-isp@FreeBSD.ORG Subject: Re: ccd and vinum Message-ID: <Pine.LNX.4.02.9901250835130.19608-100000@mercury.webnology.com> In-Reply-To: <19990125131628.C36690@freebie.lemis.com>
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On Mon, 25 Jan 1999, Greg Lehey wrote:
> I think it might be possibly better to say that you combine PPs to
> make LPs, and LPs to make LVs.
Sort of, but not quite. LVs are made up of LPs. But exactly how many PPs
are in each LP of a particular LV depends on the "mirror" property of an
LV. If it's not mirrored, there is exactly one PP per LP. If it's a
simple mirror, there are exactly two PPs for each LP in the LV. If it's a
double mirror, there are exactly three PPs for each LP in the LV. Each LP
in a particular LV has the same number of PPs as every other LP in that
LV. That's where the jump from "physical" to "logical" is made.
> In this case, a PP corresponds to a
> subdisk (but it's less flexible), an LP corresponds to a plex, and an
> LV corresponds to a volume.
I think I'm beginning to see the difference...in the LVM, PPs are used are
used as simple building blocks, all of a uniform size. I can make a LV of
any size I want (as long as it's a multiple of the PP size) by picking off
PPs from anywhere within the VG. That's also how I can grow the
filesystem, by simply adding more PPs (via LPs) to the LV.
The only thing that's got me confused about Vinum is whether or not a plex
has to exist within a single subdisk, or whether it can span subdisks, and
whether or not two plexes of a particular volume must take up the same
number of subdisks (or have roughly the same subdisk layout, at least).
> I'm not sure this is stated correctly. I'd be more inclined to
> expect: ``Mirroring is accomplished by assigning more that one LP to
> an LV''.
Nope. LVs are always made up of multiple LPs. Mirroring is accomplished by
assigning more than one PP to each LP in an LV.
Thanks for the explanation.
Cheers,
Mick
The Reverend Jasper P. O'Malley dotdot:jooji@webnology.com
Systems Administrator ringring:asktheadmiral
Webnology, LLC woowoo:http://www.webnology.com/~jooji
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