How is this possible? Hot plug connectors have uneven edge connectors so ground pins contact before signal pins or, at the very least, initialize links with in a high-Z state or have protection diodes. Did PCIe add any of these features?
Being able to hot-plug cards in/out of slots has been a thing for higher-end gear for a long time.
I'm sure the IBM Mainframe folks did it many decades ago like they've seem to have done for many things that the 'commodity' server folks are only now catching up to.
> I'm sure the IBM Mainframe folks did it many decades ago
They could do that with CPU modules a couple generations back. I don't think the newer ones can do it though, at least not a CPU at a time. It's probably possible to power off a CPU drawer (4 CPUs on the Z15, IIRC) without too much hassle.
I remember seeing a colleague of mine doing it with memory and CPUs on a live Ultra Enterprise 4000, again, with memory and CPU. IIRC, he had to tell the OS the board was going to be pulled out before doing so and a light would signal it was safe to proceed. He was doing that because we used one machine to process a lot of logs overnight (in 1997).
CPU/memory live replacement in IBM mainframes is done on per-book basis IIRC. Used to be that single book had one or two CPU complexes (often each cpu was actually two running lockstep) but these days the density went way up.
Hot-pluggableness is baked in PCI/PCI-E right in. I definitely hot plugged PCI cards on absolutely not hot-plug certified consumer motherboards. If you are not slamming it in it would work just fine. Hot unplug, on the other hand... Still, if you are on any Windows machine - just disable it in DevMgr, it would probably work just fine.
PCI-E was hot pluggable since day 1, usually you have some extra bits and pieces to make it reliable though - depending on server it might involve switches or special card holders in case or card cages which you put the card in first. And usually some status leds and "attention, I want to remove this device" button.
A card cage design was used on some IBM machines, where you attach the card a slim sled with a lever attached, this allowed you to "slide in" the card into running computer then use the lever to push it into edge connector (this also handled powering up/down the connector, iirc).
Some servers have cases which instead implement that completely in the case itself, though usually that means you need more effort to do the replacement.
Then there are alternative PCI-E connectors - ExpressCard, U.2 & SATAExpress (used in SSDs), OcuLink and related (cabled PCI-E)
Not everybody uses the PCI Express physical slot that you may be familiar with from standard PC motherboards. For example, see the OCP 3.0 card standard. They are designed logically and electrically for hot plug.
That said, hot plugging is definitely a "pets" thing and as for large scale data center computing, nobody cares. The machine fails as a unit and it's OK to take a machine offline to fix it.
Even if it worked electrically, I'd be afraid to do that on commodity home hardware while powered. The motherboard might be held in space on standoffs, and you're putting a ton of force to jam that edge card connector in. If the flex induces things in sockets to wiggle around, or poor exercises any poor solder joints, you might introduce some transient errors.
Or I'm just paranoid. A ZIF PCI-E socket would be swell for this.
How is this possible? Hot plug connectors have uneven edge connectors so ground pins contact before signal pins or, at the very least, initialize links with in a high-Z state or have protection diodes. Did PCIe add any of these features?