Question about Apache mirroring

Hi,

I'm trying to set up an Apache mirror site which would be used as a
"backup" site in case the master server is down. So if I do:

CacheRoot /home/www/http/cache
ProxyPass /mysite1backup http://www.mysite1.com
CacheSize 1024
CacheDefaultExpire 24

What happens if someone requests a page that has expired from cache,
but the master server is down? Also, is there a way to "initialize"
the cache w/o having to request each page individually?

Or maybe mirroring isn't the best way to accomplish my goal, and if it
isn't, I'd appreciate suggestions.

Thanks,

Stan McFarland