zfs on root and zpool.cache importance

[Michal]

Hello

I'm a bit confused about zpool.cache file. I've got a configuration with 
/boot sitting on a usb drive (UFS) and everything else on internal ZFS 
hard drive. I'm booting my system off the usb drive so zpool.cache file 
is there (usb drive). Basic zfs root + ufs boot setup. Everything works 
like a charm no problems so far and I would like to keep it that way 
hence my question.

I've noticed that zpool.cache on ZFS drive is being updated from time to 
time and it is different from zpool.cache file on usb drive. Even when I 
remove zpool.cache file on hard disk then it gets recreated 
automatically and system still boots and works fine because it starts 
with zpool.cache on usb drive which is intact.
Now how important it is to keep them in sync and what I'm risking by not 
doing that? Am I stomping on a thin ice? Should I copy zpool.cache from 
hard drive to usb boot disk every day or should I leave it how it is?

I know that zpool.cache is critical to boot and it keeps some 
informations about pool configuration but don't quite understand 
implications of having it outside of the pool itself (without zfs even 
knowing about that).

Michal
-- 
"Every man dies, not every man really lives." -William Wallace