Following the Dyn outage on Friday we are considering adding a secondary authoritative DNS provider. We would like to understand the real-world impact should one of the providers have an outage.
For example, if our NS records were to look like
ns1.provider-a.com
ns1.provider-b.com
ns2.provider-a.com
ns2.provider-b.com
and either provider-a or provider-b experienced an outage, what would users experience in the worst case (no cache)? I would expect something like increased latency getting a valid response (should the resolver first attempt to reach a downed server), or perhaps a resolution failure 50% of the time. If the behavior is implementation-dependent, any understanding of what the spread of various behaviors is would be very helpful.