UPDATE: The test is complete.
tl;dr; Planned service interruption that will impact all Stack Exchange sites, Jobs, and Chat. All sites will be read-only for up to 1 hour on Wed June 30, 2021 at 01:00 UTC (Tue June 29 9:00 PM US/EDT). Stack Overflow might experience some instability during this time as well. Enterprise cloud hosted instances will not be impacted.
There will be a service degradation for up to 1 hour, 01:00 UTC on June 30th, 2021 (June 29th 9:00 PM US/EDT). During that time questions and answers will still display, job listings will still work, and job ads will still display. However, the site will be "read only," i.e. people won't be able to add/edit new job listings, apply for jobs, post, edit or vote on questions/comments/answers, reputation won't change, etc. This should minimize the disruption to the majority of casual readers. We will display a banner on the sites stating we're 'read only' for maintenance.
Longer Version of What's Taking Place?
Our primary database servers, which power Stack Overflow, Stack Exchange, Chat, and other things, run out of a datacenter in New Jersey plus our secondary location in Colorado. We are performing a test of our disaster recovery system as part of our regular system maintenance.
What we'll be doing
During the service interruption traffic will be directed to our secondary location in Colorado, but services will remain in read-only mode. This will allow us to confirm a number of systems are running properly, while under a full production load. Once we’ve had an opportunity to examine our metrics and logs, we will move the traffic back to the primary data center.
Questions or concerns?
Please post a comment or answer below; I'll do my best to address any concerns between now and the maintenance window.