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Date: Mon, 24 Feb 2014 12:26:04 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: Chris Stankevitz <chrisstankevitz@gmail.com>
Subject: Re: gptids are not symlinks to /dev/da*
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On Mon, 24 Feb 2014, Chris Stankevitz wrote:

> Hello,
>
> Can you help fill in some detail for the questions I list below?  Thank you!
>
>
> Facts:
>
> 1. zpool status shows "gptids" like so:
> gptid/affd7a1f-9c50-11e3-a81e-002590caf078
>
> 2. gptids are located under /dev/gptid/ and /dev
>
> 3. smartctl does not like gptids but does like /dev/da* "smartctl -a
> /dev/gptid/affd7a1f-9c50-11e3-a81e-002590caf078"
>
> 4. "man gptid" comes up blank
>
> 5. the freebsd handbook does not describe gptid
>
> 6. google "freebsd gptid" returns a bunch of random stuff
>
>
> Questions:
>
> Q1: What is a gptid?
>
> A1: It's some kind of system by which drives have the same device name
> across reboots

Not device name, but a label.  Although the correct one should work in 
place of a device name.  I use GPT partition labels, but have never 
bothered with the GUIDs.

> Q2: How does FreeBSD know which drive corresponds to which gptid?
> When is the gptid assigned?
>
> A2: gptids are written to the first block of the disk during manufacture.

Generated when the structure is created, I think.

> Q3: Can I use it like a regular /dev/da? device?
>
> A3: No.  For example, smartctl takes /dev/da* but not /dev/gptid/*

smartctl wants a device, but GPTIDs refer to partitions.  There is a 
GUID for the whole disk, but it appears to not be shown.

> Q5: Why am I unable to figure out what is a gptid even after
> attempting to read documentation/google?

GPT IDs and labels are created by the geom_label class, described in 
glabel(8).