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To: Warren Block <wblock@wonkity.com>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Fri, 07 Oct 2011 17:41:59 CST."
	<alpine.BSF.2.00.1110071734140.4137@wonkity.com>
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Date: Sat, 08 Oct 2011 08:18:54 +0000
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Cc: freebsd-current@freebsd.org, Garrett Cooper <yanegomi@gmail.com>,
	Glen Barber <gjb@freebsd.org>, "Andrey V. Elsukov" <bu7cher@yandex.ru>,
	Benjamin Kaduk <kaduk@mit.edu>, Arnaud Lacombe <lacombar@gmail.com>
Subject: Re: aliasing (or renaming) kern.geom.debugflags
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In message <alpine.BSF.2.00.1110071734140.4137@wonkity.com>, Warren Block write
s:

>Since we're talking about this, could you review the usage in the 
>gmirror section of the Handbook GEOM chapter:
>   http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom-mirror.html
>
>Seems like that is a valid non-debugging use, to allow the last block to 
>be written.

gmirror and this procedure has several problems:

1. It steals the last sector on the disk.  If that sector contained data
   you lost them, with no notice.  Most often it will not, particularly
   on a freshly installed system, but it is still a bad thing.

2. The paritioning is not fixed up to record the stealing of this sector.
   I wouldn't be surprised if this could cause confusion down the road.

3. In this case, writing only happens to a single sector, which we assume
   is not going to be written by anybody else, so apart from #1 and #2
   debugflags=16 does not cause any additional damage.

This is the kind of usage that makes me sad I ever added that option.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.