linux-kernel - Re: dm stripe: Fix bounds

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Date:	Fri, 13 Oct 2006 23:22:37 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Alasdair G Kergon <agk@...hat.com>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org,
	Heinz Mauelshagen <mauelshagen@...hat.com>
Subject: Re: dm stripe: Fix bounds

On Thu, Oct 12, 2006 at 04:47:59PM -0400, Phillip Susi wrote:
> One stripe table can only contain one stripe size, so to have two would 
> require two tables, and a third table to tie them back together.

One device-mapper table can contain two concatenated stripe targets.

  http://people.redhat.com/agk/talks/FOSDEM_2005/text6.html
 
> The entire idea of a stripe is that you are using multiple identical 
> drives ( or partitions ), so it doesn't make any sense to be able to 
> truncate one of the drives.  

dmraid is not the only user of device-mapper striping.  Userspace volume
managers may want to use all sorts of odd layouts quite legitimately.

> In any case, this is not something you can 
> do now, 

[Actually that's what the code did before this patch:-(]

> so the fact that you could not do it then either does not seem 
> to be a good argument against allowing partial tails.
 
The arguments are (1) to avoid the ambiguity I've discussed and (2) to avoid
the additional complexity the in-kernel striped target would require
(calculating a stripe size for the end of the device to override the one
supplied), when it's so simple for userspace to specify exactly what it
requires by using two targets.

Alasdair
-- 
agk@...hat.com
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