Date: Tue, 26 Jan 1999 08:47:30 -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.4.02.9901260840530.7880-100000@mercury.webnology.com> In-Reply-To: <19990126122753.B66239@freebie.lemis.com>
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On Tue, 26 Jan 1999, Greg Lehey wrote:
> Ah. What you're saying is that each LP also has the same storage size
> as a PP (4 MB), and that you're combining a number of them to make up
> the size of the LV?
Bingo.
> OK. We're looking at this from a different side. If I've understood
> you correctly, a 16 MB mirrored LV would look like:
[diagram snipped]
Yes, that's it.
> In Vinum, a 16 MB mirrored volume made of 4 MB subdisks might look
> like this:
Ahhhhhhhhh. I think I've got it.
> The result is the same, but the concept of plexes is pretty much the
> opposite of the concept of LPs. In particular, I don't see how you
> can stripe effectively with the LVM approach.
It's bloody weird, actually. I've never quite understood it myself.
Basically, the LVM breaks up the the PP into even smaller blocks, and
stripes across PPs with those itty-bitty blocks:
PP1-IBB1 --> PP2-IBB1 --> PP3-IBB1 --> PP1-IBB2 --> PP2-IBB2 --> etc.
where IBB stands for "itty-bitty block," of course.
> This diagram should also answer your question above: plexes contain
> one or more 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).
>
> No. There's no constraint there.
So I can have one plex contained entirely within a single subdisk, and a
second plex spread across two (or more) subdisks?
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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