Stumped by gpart and gmirror, trying to set up RAID1 (10.0-Beta3)

[Anton Sayetsky]

2013/11/23 Mike. <the.lists at mgm51.com>:
{...}
> [root at a20m /etc]# gpart show mirror/gm0
> =>      1  1981438  mirror/gm0  MBR  (967M)
>         1       31              - free -  (16K)
>        32  1981376           1  freebsd  (967M)
>   1981408       31              - free -  (16K)
{...}
> [root at a20m /etc]# gpart add -t freebsd-ufs  -a 4k -s 2g
> mirror/gm0s1
> gpart: autofill: No space left on device
So you're trying create an 2G-sized partition inside a slice of 967
mebibytes. Why'd you think that it should work?