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