how to change provider for next boot?

[Ferruccio Zamuner]

Hi,

I've a gmirror degraded on ad4s1 and ad4s1 has some error:

Jan 31 16:36:06 r1 kernel: ad4: FAILURE - READ_DMA 
status=51<READY,DSC,ERROR> error=40<UNCORRECTABLE> LBA=43164095
Jan 31 16:36:06 r1 kernel: GEOM_MIRROR: Request failed (error=5). 
ad4s1[READ(offset=22099918848, length=131072)]

ad6s1 was in mirror with ad4s1, how can I tell to the system to boot 
from ad6s1next time?

Thank you in advance.

Bye,                 \ferz