7.3-RC2 - fdisk cannot write or modify sector zero on gmirror - GEOM bug?

[Miroslav Lachman]

I sent this message with subject "Geom not found: "gm0" / Failed to 
write sector zero" to the freebsd-stable list a week ago, but nobody 
replied. I did some tests - more on the end of this message.

---------------------------------------------------
I just installed 7.3-RC2 amd64 on new server.

I created slice s1 (80GB on disk ad4 (500GB), then partitions for system 
(/, swap, /var, /usr, /tmp) by sysinstall.

After base install I created gmirror gm0 as usual (I did it many times).

Now I am no longer in datacenter and have only ssh access to this server 
and I need to create slice s2 with some partitions for data storage, but 
fdisk failed.

     fdisk -u /dev/mirror/gm0

At the end, I got this error:

     Should we write new partition table? [n] y
     fdisk: Geom not found: "gm0"
     fdisk: Failed to write sector zero

Fdisk failed even if I used

     sysctl kern.geom.debugflags=16


Question #1 - why 'Geom not found: "gm0"'?

Question #2 - is there any way to create slices + partitions on unused 
space if system is booted from this device?
Or is the only way to boot it from some LiveFS / fixit?


I found the same question on this list, but without reply
http://lists.freebsd.org/pipermail/freebsd-stable/2009-June/050855.html

I hope somebody can help / explain it.

---------------------------------------------------

Some newer tests.

Today I tried it with 7.2 RELEASE under QEMU without gmirror and there 
is not any problem to write sector zero on to /dev/ad0

I can resize ad0s1 by fdisk with sysctl kern.geom.debugflags=16 (system 
is booted from ad0s1a) and I can create ad0s2 without any problem.

But this is impossible on gmirror with 7.3-RC2. It seems as some bug in 
GEOM / gmirror for me.

Can somebody with GEOM knowledge take a look at it?

Miroslav Lachman