Since I upgraded, I am getting messages like this one

 WARNING: PV /dev/sdb2 in VG chiark-early-b is using an old PV header, modify the VG to update.

I have read many docs and web pages all of which seem to suggest that
I might want to do something like "vgck --updatemetadata".

However, I can find no documentation which allows me to establish
whether this is safe to use in my use case: I need my LVM PVs to be
readable even with certain older versions of the LVM tools.

The message "modify the VG to update" is also concerning.  Does that
mean that the PV metadata might be automatically updated ?  Is there a
way to prevent this to avoid making my volumes unreadable by older
software ?

I see the following possibilities:

 * Updating is always done but the new format is always readable by
   older tools.  In this case, please treat this bug report as a
   request for this situation to be documented.

 * Updating is not always done, because the new format is not always
   readable by older tools.  In that case, please treat this bug
   report as a request for:
     - documentation of which older versions are able to read
       the newer format(s);
     - a way to suppress the message, for use if the administrator
       finds they cannot safely upgrade.

 * Updating is always done BUT the new format is not always readable
   by older tools.  This would seem like a bug of severity "serious"
   at the very least since it might at least lead to very serious and
   unexpected inconvenience (eg in the context of emergency recovery
   situations, or backup restores).

Thanks,
Ian.