> My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter.
Google have a variant on this, where they use a GPS receiver in each data centre to provide an accurate time source for local machines.
No, they use a GPS with 7ms time spread for servers as you said. What Im describing is a custom device set against an atomic clock to nano or microsecond accuracy which then does the same to the servers via low latency interconnects. Optionally with time server & dedicated networks for reduced admin overhead.
Should do a lot better than 7ms with performance implications.
Google have a variant on this, where they use a GPS receiver in each data centre to provide an accurate time source for local machines.