[Bug 294326] glabel.8: Document conditions where (automatic) labeled devices aren't created

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=294326

            Bug ID: 294326
           Summary: glabel.8: Document conditions where (automatic)
                    labeled devices aren't created
           Product: Base System
           Version: 16.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: michaelo@FreeBSD.org

I wanted to make use of automatical disk labels /dev/diskid/ for ZFS and
failed. Consider the follwing configuration:
root@deblndw013x:~
# camcontrol inquiry da0
pass0: <HP RAID 1(1+0) OK> Fixed Direct Access SPC-3 SCSI device
pass0: Serial Number 50014380147AB8F0
pass0: 135.168MB/s transfers, Command Queueing Enabled
root@deblndw013x:~
# camcontrol inquiry da1
pass1: <HP RAID 5 OK> Fixed Direct Access SPC-3 SCSI device
pass1: Serial Number 50014380147AB8F0
pass1: 135.168MB/s transfers, Command Queueing Enabled
root@deblndw013x:~
# camcontrol inquiry da2
pass2: <HP RAID 1(1+0) OK> Fixed Direct Access SPC-3 SCSI device
pass2: Serial Number 50014380147AB8F0
pass2: 135.168MB/s transfers, Command Queueing Enabled

Note: Please don't ask why it is RAID, the controller firmware does not support
HBA mode.

As you can see the S/N is identical for all three VDs (see also
https://www.claudiokuenzler.com/blog/413/freebsd-cciss-hp-smart-array-raid-wrong-disk-numbering-labeling).
Whether this is a firmware bug in the controller or in ciss(4) I don't know,
but this can happens with GPT labels and other automatic labels.

I ended up with only one labeled device. dmesg didn't show a WARNING/ERROR
nothing. Looking into sys/geom/label/g_label.c I see g_label_create() three
conditions:
* Label invalid. Not documented what characters are permissive.
* Label too long. 64 chars including prefix and null byte.
* Label already exists, first wins. My case.

I believe that they should be part of the manpage especially for the
automatically created labeled devices.

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