Recently I had a design session with a customer looking to setup a fairly unique DR scenario. They are a health care organization with 2 central datacenters located within the same town 10 miles apart.
Additionally they had hundreds of clinics and offices scattered across the state that had no DR plans in place. A quick solution was to setup vSphere Replication between these external locations and the primary DC in the HQ. But the customer wanted an additional layer of security.
The proposal was to setup a vSAN stretched cluster between the primary and secondary DC’s. Using nested fault tolerance each DC would have FTT=2 while using FTT=1 between the DC’s.
This would give the customer a primary target for all external DC’s giving them FTT=2. Additionally the primary DC was replicating to a secondary DC which also enabled FTT=2. This gave the customer resiliency both within a site and between sites.
This was only possible due to the low latency between primary and secondary sites so if you consider something like this please keep in mind the stretch vSAN cluster requirements:
Data Site to Data Site Network Latency
Data site to data site network refers to the communication between non-witness sites, in other words, sites that run virtual machines and hold virtual machine data. Latency or RTT (Round Trip Time) between sites hosting virtual machine objects should not be greater than 5msec (< 2.5msec one-way).
Finally the customer elected to host the witness node at a non-production test facility in another city. Luckily the networking and site latency requirements for this are much more lenient:
Data Site to Witness Network Latency
This refers to the communication between non-witness sites and the witness site.
In most vSAN Stretched luster configurations, latency or RTT (Round Trip Time) between sites hosting VM objects and the witness nodes should not be greater than 200msec (100msec one-way).
In my next post I’ll go over some interesting behavior we discovered with vSphere Replication with regard to FTT settings and vSAN.