[palmotea]

Before 9/11, most DR (disaster recovery) sites were in Jersey City, NJ just across the river from their main offices in WFC or WTC, or roughly 3-5 miles away. After 9/11, the financial industry adopted a 50+ miles rule.

IIRC, multiple IBM mainframes can be setup so they run and are administered as a single system for DR, but there are distance limits.

[linksnapzz]

A Geographically-Dispersed Parallel Sysplex for z\OS mainframes, which IBM has been selling since the '90s, can have redundancy out to about 120 miles.

At a former employer, we used a datacenter in East Brunswick NJ that had mainframes in sysplex with partners in lower manhattan.

[sllabres]

If you have to mirror synchronously the _maximum_ distances for other systems (e.g. storage mirroring with NetApp SnapMirror Synchronous, IBM PPRC, EMC SRDF/S) are all in this range.

But an important factor is, that performance will degrade with every microsecond latency added as the active node for the transaction will have to wait for the acknowledgement of the mirror node (~2*RTT). You can mirror synchronously that distance, but the question is if you can accept the impact.

That's not to say that one shouldn't create a replica in this case. If necessary, synchronize synchronous to a nearby DC and asynchrone to a remote one.

For sure we only know the sad consequences.

[linksnapzz]

The actual distance involved in the case of the Brunswick DC is closer to 25 miles to Wall St.; but yes, latency for this is always paramount.