Simple way to clear arbitrary drive metadata?

[Konstantin Kukushkin]

29.09.2012 21:27, Warren Block пишет:
> On Fri, 28 Sep 2012, Xin Li wrote:
>
>> On 09/28/12 15:21, Warren Block wrote:
>>> Last night, I found that the remnants of a GPT backup table on an
>>> MBR drive prevented it from booting.  When reusing drives from old
>>> mirrors, old mirror metadata can be a problem also.  And there may
>>> be old hardware RAID metadata at the end of the drive.
>>>
>>> It would be great if dd understood negative seek values.  This
>>> would get most of that old metadata:
>>>
>>> dd if=/dev/zero of=/dev/ada8 seek=-34
>>>
>>> ...but dd does not understand negative seek values.  (Been on my
>>> list for a while to look at that.)
>>>
>>> Which leaves things like
>>>
>>> diskinfo ada8 | cut -f4 (subtract 34) dd if=/dev/zero of=/dev/ada8
>>> seek=(calculated value)
>>>
>>> That can be done in one command line with bc and backticks, but
>>> it's not clear or elegant.  gpart can clear secondary GPT tables,
>>> but I'm pretty sure it won't wipe out that space unless it actually
>>> is a GPT table. Likewise with glabel and gmirror, they're safe
>>> because they only touch data they understand.
>>>
>>> Is there something simpler and more blunt?
>>
>> I think you can do:
>>
>> gpart destroy -F ada8
>> gpart create -s gpt ada8
>> gpart destroy -F ada8
>>
>> The second 'create' will write an empty partition table to the
>> secondary table.
>
> Nice!  It works perfectly for GPT and glabel metadata.
>
> gmirror is a problem.  If the gmirror kernel module is loaded, drives
> with gmirror metadata create a mirror.  GEOM prevents writes to the
> drive then.  sysctl kern.geom.debugflags=16 allows writes, but the
> mirror is still in memory and running.  'gmirror stop' (which the system
> also does on shutdown) helpfully writes the whole metadata block back to
> the drive.  After reboot, it's right back where it was.
>
> Seems like the only way to deal with gmirror is to have the user check
> for it directly, and stop the mirror if the attached drive is a member.

You may use 'geom <class> clear' command to clear metadata from 
disk/partition/other GEOM provider. This will work only on stopped 
mirrors or while geom_mirror not loaded, of course.
For gmirror you can 'remove' component from running mirror (even if the 
component is last). Metadata will cleared after removing, so gmirror 
remove <mirror_name> <component_name>' is very usable.

You can determine what GEOMs metadata living on disk $disk using command 
'geom <class> dump', like this:
for class in mirror stripe concat raid3; do
   geom $class dump $disk
done

> Probably the same for graid, but I don't know if I have a system where I
> can test that.
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С уважением, Константин Кукушкин,
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