diskid documentation

[Warren Block]

On Mon, 2 Jun 2014, Michael W. Lucas wrote:

> On Mon, Jun 02, 2014 at 10:45:52AM -0400, Ryan Stone wrote:
>> On Mon, Jun 2, 2014 at 9:26 AM, Allan Jude <allanjude at freebsd.org> wrote:
>>> It also tends to sometimes hide the gpt label provider on me (not sure
>>> in which cases it does this, but it is annoying)
>>
>> This happens when something (e.g. zfs) happens to open the diskid
>> provider instead of the gpt label.  For me this ended up being a bit
>> more than annoying; my swap was mounted in /etc/fstab via a gpt label
>> so I silently lost my swap when I did an upgrade.
>
> Wait-- one type of one label can hide another?
>
> I thought a big point of labels was to remove ambiguity...

Can't get more unambiguous than only having one label!

The word to look for here is "wither".  When a device label name is 
opened exclusively (like mounting a device), other labels for that 
device are supposed to "wither" and disappear from view.

(My understanding of this is incomplete.  Garrett Wollman sent me a very 
nice explanation of how this works, which I have read but not enough 
times yet.)