Google recommends adding link-rel-canonical tags for pages with duplicate content, but before this was the standard, duplicate pages are used for mirroring websites to ensure information availability in case the main server fails. Say, I have set all my mirror sites to point to pages in the main site as their canonical link, and for some reason the main site went down, will Google detect that and display a mirror link in SERP, or all the mirror pages sink down along with the dead canonical?
I recognize that it is easy to update canonical link on sites that support server-side scripting but for back-ups of a static website it is tedious to update each page's link-rel-canonical tag one-by-one.
