Migrations and parallel work.
WHERE THIS ARISES
Migrating between servers Comparing a working machine with a broken one Copying configuration
WHAT TO ESTABLISH FIRST
Which is which, unmistakably.
HOW
Different terminal colours, or a prompt showing the hostname clearly.
WHY
Running a destructive command on the wrong machine is a common and severe mistake.
WHAT TO USE FOR COPYING
rsync over SSH, directly between them where possible.
WHAT TO DO WHEN THEY CANNOT REACH EACH OTHER
Copy via your own machine, or a shared location.
WHAT TO COMPARE
Package lists Configuration files Directory listings with sizes
HOW TO COMPARE FILES ACROSS MACHINES
Produce checksums on each and compare the lists.
WHY CHECKSUMS
Sizes match while contents differ, more often than expected.
WHAT TO DO BEFORE CUTTING OVER
Confirm the new machine serves the sites, by testing against it directly.
HOW TO TEST BEFORE DNS CHANGES
Override the address locally in your own hosts file.
WHY THAT IS THE MOST USEFUL MIGRATION TECHNIQUE
It proves the destination works before any customer is affected.
WHAT TO KEEP
The old server, intact, until you are certain.
