"gpart add" falsely claiming "No space left on device"

[Warren Block]

On Tue, 6 Sep 2016, Perry Hutchison wrote:

> I copied the 10.3-RELEASE memstick.img to a 4GB flash drive, then used
> "gpart recover" to resize the partition table to the media.  After that
> "gpart show" reports:
>
> # gpart show da2
> =>      3  7811067  da2  GPT  (3.7G)
>        3       32    1  freebsd-boot  (16K)
>       35  1348832    2  freebsd-ufs  (659M)
>  1348867     2048    3  freebsd-swap  (1.0M)
>  1350915  6460155       - free -  (3.1G)
>
> but "gpart add" refuses to add a second freebsd-ufs partition in that
> supposedly-free space:
>
> # gpart add -t freebsd-ufs -l pkgs -f x da2
> gpart: index '4': No space left on device
>
> # gpart add -t freebsd-ufs -l pkgs -f x -b 1350915 -s 6460155 da2
> gpart: index '4': No space left on device

The second one makes more sense, as the first '-f x' would/should have 
allocated that space (in an uncommitted operation).  Don't know about 
the first one, unless you have tried it before.

Why bother with '-f x'?  Why not just do the operation immediately?