question regarding geom labels

[Warren Block]

On Fri, 30 Mar 2012, Alexander Best wrote:

> i have a question regarding a label for a swap partition. when should i do the
> labeling? after or before creating the partition scheme?
>
> when i label before creating the partition scheme, likes this:
>
> glabel label -v swap /dev/da0
> gpart create -s GPT /dev/da0
>
> i get the following warning:
>
> GEOM: da1: the secondary GPT header is not in the last LBA.
>
> which is obvious, because the label is being written into the last LBA and thus
> the backup GPT header gets written into the last-1 LBA.

Right.  Don't do that, the GPT backup header needs to be at the end of 
the physical device.  If you're using that whole disk for swap, there's 
no need for a partition anyway.

> if i create the partitioning scheme before labeling the device, like this:
>
> gpart create -s GPT /dev/da0
> glabel label -v swap /dev/da0
>
> or
>
> gpart create -s GPT /dev/da0
> gpart add -t freebsd-swap /dev/da0
> glabel label -v swap /dev/da0p1
>
> the label gets written into da0 or da0p1 and is at constant risk of being
> overwritten by userdata.

No.  The swap device entered in /etc/fstab would be /dev/label/swap, 
which is one block smaller than da0p1.  That's the last-block metadata, 
it's safe.

But if the whole disk is for swap, skip the partitioning entirely.