Why fdisk can't open root disk with MBR for writing?

[Lokadamus]

Am 16.09.2012 11:41, schrieb Chris Rees:
> On 16 September 2012 10:23, Julian H. Stacey <jhs at berklix.com> wrote:
>> This is a resend as Benjamin Kaduk <kaduk at mit.edu>
>> dropped the
>>          Yuri <yuri at rawbw.com>
>> from CC line, & Yuri was the original first poster in thread who
>> my patch would presumably have helped.
>>
>> Reference:
>>> From:         Benjamin Kaduk <kaduk at mit.edu>
>>> Date:         Sat, 15 Sep 2012 14:49:41 -0400 (EDT)
>>> Message-id:   <alpine.GSO.1.10.1209151448200.861 at multics.mit.edu>
>> Benjamin Kaduk wrote:
>>> On Sat, 15 Sep 2012, Julian H. Stacey wrote:
>>>
>>>> I have been applying this diff to my man fdisk:
>>>>
>>>> http://berklix.com/~jhs/src/bsd/fixes/FreeBSD/src/gen/sbin/fdisk/
>>>>
>>>> *** 8.0-RELEASE/src/sbin/fdisk/fdisk.8      Sat Mar 14 22:32:16 2009
>>>> --- new-generic/src/sbin/fdisk/fdisk.8      Sat Mar 14 22:35:10 2009
>>>> ***************
>>>> *** 462,464 ****
>>>> --- 462,468 ----
>>>>   The
>>>>   .Xr bsdlabel 8
>>>>   command must be used for this.
>>>> + .Pp
>>>> + When running multi user, you cannot write unless you first run this:
>>>> + .br
>>>> + sysctl kern.geom.debugflags=16
>>>>
>>>> I never submitted it as a send-pr,
>>>> anyone think I should submit it to help save people ?
>>> We have had a long discussion about kern.geom.debugflags starting here:
>>> http://lists.freebsd.org/pipermail/freebsd-current/2011-October/028090.html
>>> My understanding from that discussion is that your patch should not be
>>> accepted.
>> Got to travel now, will read that thread later,
>>
>> A shame if we would leave fdisk crippled undocumented, when so easy
>> to doc. the solution. Fdisk is known across many OSs Unix & beyond,
>> our BSD tools may be better nicer, but fdisk it what many will first
>> reach for,
>>   a shame not to help newer people & visitors used to fdisk.
>> We inside FreeBSD crippled fdisk by adding these flags. Outsiders
>> wont expect the weirdness & get impression FreeBSD has a bug.
> I think you have a point, but at the moment fdisk really doesn't work
> properly at all; I find so many people complaining on IRC about "Why
> doesn't fdisk work?"
>
> To be honest, I'd be happy with replacing fdisk with a huge warning
> "USE GPART!!"; fdisk isn't really standard anyway.
>
> The alternative of course is to fix fdisk...  *properly*.
>
> Chris
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FreeBSD 9.1RC1 have a live-cd option and with this, fdisk work fine.