So they didn't have:
Some kind of HDR (high availability server) to seamlessly swap over to?
A SDS secondary shared disk server to failover to?
An Enterprise replication machine somewhere?
A RSS Remote standby machine, in another location or even the cloud?
Probably Only Informix does this well (and would give me some work), Oracle tries but it's problematic.
I wonder if it were database / application or what. Doesn't sound like a network Issue. SANs are bulletproof as well. I would assume there is a more sinister reason given the state of banking.
Mainframe - no wonder it fails, like almost nobody is alive who knows how to maintain the OS. I've tried and it's super command line unfriendly.
Banks are sadly expertless, I know loads with terrible DBA's runing mickey mouse systems with no failover. The more you know the more you wonder how it works AT ALL.