Dangerously Dedicated

James Phillips anti_spam256 at yahoo.ca
Thu Dec 10 08:04:30 UTC 2009


> Date: Thu, 10 Dec 2009 03:12:45 +0100
> From: Polytropon <freebsd at edvax.de>
> Subject: Re: Dangerously Dedicated
> To: Maxim Khitrov <mkhitrov at gmail.com>
> Cc: freebsd-questions at freebsd.org
> Message-ID: <20091210031245.3fd58187.freebsd at edvax.de>
> Content-Type: text/plain; charset=US-ASCII
> 
> On Wed, 9 Dec 2009 20:41:40 -0500, Maxim Khitrov 
<mkhitrov at gmail.com> > wrote:
> > On Wed, Dec 9, 2009 at 8:33 PM, Rolf Nielsen
> > <listreader at lazlarlyricon.com>
> wrote:
> > > As far as I understand it, it's called
> Dangerously Dedicated because it may
> > > cause other systems not to recognise the disk.
> Consequently, newfs'ing a
> > > slice without first partitioning it can hardly be
> DD, since that is what
> > > other systems do, right?

I think I understand: using the DOS compatible partition (slice) 
table follows the principle of "least surprise." That is why I use 
"slices" for my dedicated BSD machine. 4 places to put your data are 
ostensibly better than 1, and I avoid any possible BIOS bugs if the 
BIOS sees a "non-standard" MBR. 

> > 
> > That is correct. That slice will not be bootable, but
> you can use it
> > to store data.
> 
> Being bootable is a matter of what the MBR boot block
> says. In a DD setting, it refers to the first partition
> (that's not within a slice), e. g. ad0a. Especially in
> a multi-OS setting, the use of slices seems to be
> strongly recommended so all operating systems behave
> in the required way (due to compatibility reasons,
> see "DOS primary partitions"), which limits the number
> of slices to 4.

I would say a common partition format is REQUIRED in a multi-boot 
situation. For PC OS's, that means DOS compatibility.

 
> For plain storage, it's not needed to encapsulate the
> partition with the file system inside a slice, e. g.
> 
>     ad1  ad1s1  ad1s1e
>     {    [     
> (/data)  ]  }
> 
> in comparison to
> 
>     ad1  ad1c
>     {    (/data)  }
> 
> And as it is known, the "c" can be omitted, as in
> 
>     # mount /dev/ad1 /data
> 
> 

The Detailed 8.0 release notes don't say anything about bootability:
        2.2.5 File Systems
          “dangerously dedicated” mode for the UFS file system 
        is no longer supported
 http://www.freebsd.org/releases/8.0R/relnotes-detailed.html

I also note that the DOS partition (slice) table is not explictly 
required either: could you use an Apple partiton (slice) table 
instead? 
UFS not supporting DD mode struck me as weird BECAUSE it has to work 
with different architectures. 
Of course, if you are just storing raw data, you don't always *need* 
a filesystem. 

Regards,

James Phillips


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