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Comment by rubyn00bie | Hacker News Reader

Well... sorry for the tangent, but this blew my mind (totally knew about pcie and disks):

> On systems with hot-swappable RAM, memoryN/state might be “offline”,

Anyone have any information on this or real world use cases not better suited for having a cluster or multiple redundant machines? Are these still prevalent in some markets 2020? Looks like, according to really bad results in teh googs, CPUS are hot-swappable too? Is it a an extra socket on the motherboard or like some sort of franken dual-board setup?

I'm more curious than anything how systems like that are used or what their niche is... I'm sure they're insanely impractical for almost any conceivable purpose but what are those few cases they aren't? What's it like swapping out a CPU or DIMM in those? Just a lot of holding onto your butt and praying?

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Hot-swappable RAM was and is a standard feature on mainframe equipment (along with often hot-swappable CPUs), and was also a common feature on higher-end server equipment through the '90s (for example Sun). It also appeared of course in some of the more in-between architectures such as HP Tandem/NonStop, which also featured hot-swappable CPUs. It was not unusual on these machines to have flashing status lights which would lead a technician to which drawer to open and which module in that drawer to swap out.

There wasn't a particular niche for this equipment, it was just intended to be repairable while in service. At the time, it was typical to spend a larger dollar amount on equipment which was manufactured for very high serviceability. A combination of improvement in the actual reliability of hardware, increasing cost pressure, and probably as the lesser factor a shift towards more horizontal architectures (where redundancy was across machines instead of across components in machines) has made this a lot less common, although still not at all unheard of on high-end equipment.

I've upgraded both RAM and CPUs on IBM RS/6000 machines (pSeries these days I believe) while the system was running. Very nice capability to minimize downtime. The procedure is 100% supported. You start by telling the OS (AiX in the cases where I did this) that you are going to remove these RAM modules or these CPUs and the OS let's you know when it has moved everything off of those. Then you remove them and plug in the replacement, then tell the OS that the new hardware can be used. Very smooth process.

Though I think on Linux it was added for VMs, so you can scale those up and down without rebooting.

Banks and airline reservation systems tend to use vertically scaled "mainframe" architectures. In this model you need very high availability so hot-swappability is important. Linux even has hot-swap CPU support for systems like this [1]. Some IBM systems have dozens of hot-swappable hardware elements. Power supplies, CPUs, network cards, RAM, disks.

[1] https://www.kernel.org/doc/html/latest/core-api/cpu_hotplug....

Taking memory offline is pretty common, that’s part of how “chipkill” ECC works. Servers that are even slightly fancy have mirroring support with a spare for every module. HPE ProLiant servers have this feature and they’re just rack mounted PCs.

Actually removing and replacing these parts while the system runs requires electrical and mechanical features that most machine lack.

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