Change processor and memory without shutting a machine down

On the previous virtualisation platform, processors and memory could be added to a running machine without stopping it. That capability was lost in the move to the new platform and had to be rebuilt.

What was missing

Two separate halves, and either one alone does nothing. The platform excludes processor and memory from live changes unless explicitly enabled. Beyond that, the guest operating system ships a rule that brings newly-appeared processors online only on two other virtualisation platforms — under this one it quietly does nothing, so hot-added processors would have appeared and then sat unused.

Result

Both halves are in place and proven on a live machine in both directions — processors added and removed again while it kept serving, with no restart. A machine's maximum is fixed when it starts, so raising the ceiling still needs a restart; changes within that ceiling are now live.

The ceiling was applied across the estate with a rolling restart of all fifteen Linux machines, one at a time, each drained before and returned to service after.