Now, exactly what form that hardware takes is an open question. I would assume it's something like "NVME interface hardware" + "ARM CPU which implements the EBS protocol" + "25 GbE PHY", but that guess is based solely on "that's how I would design it".
> There's absolutely no way that they would get the performance I'm seeing from an emulated disk. We're talking to real hardware, exposed via PCI passthrough.
There's a wide spectrum between "emulated" and "real hardware, exposed via PCI passthrough". Passing through to PCI hardware, in and of itself, gains you very little in terms of absolute guest-visible performance versus eliding all VMEXITs via other means, but it has other important characteristics that I suspect AWS very much wants in the c5 family.
> I would assume it's something like "NVME interface hardware" + "ARM CPU which implements the EBS protocol" + "25 GbE PHY", but that guess is based solely on "that's how I would design it".
I would expect something along these lines, although I'd be a little surprised if they bothered putting the NVMe bits down in silicon. My personal guess would be a good silicon DMA engine and PCIe interface married to sufficient general-purpose processing (ARM SoC, FPGA, etc.) to keep pace with the NVMe Command/Completion queues.
Regardless of how they've implemented it, the end-to-end result seems to hang together very nicely. Kudos to the team at AWS.
(note: I work on Google Compute Engine's hypervisor; my speculation about AWS really is speculation about how they'd do this — the two companies have very different engineering approaches, so I may be entirely wrong trying to project onto theirs — like I said at the top, it's fun to try :)
You and Colin both know they bought Annapurna Labs, right?
We don't have to speculate _that_ much about what is probably going on here...
I guessed a hardware NVMe interface because that seems like something which could be acquired more or less off-the-shelf, thus minimizing the engineering risks.
(Colin's speculation in a peer reply is also reasonable -- personally I've seen enough errata in "off the shelf" IP to shudder at the idea of anything in silicon that doesn't have to be, but fundamentally I'm a SWE, so that would be my take, wouldn't it)
Honestly, I'm mostly curious about how much of "KVM" you're running that's stock, how much is modified, and how much of the userland is running on the far side of PCIe rather than in host ring3 (particularly given "C5 instances are built using a new light-weight hypervisor, which provides practically all of the compute and memory resources to customers’ instances.").
[0]: Especially this one: https://news.ycombinator.com/item?id=15641391
"but the latest generation EC2 instances offload networking and storage processing to hardware. This is the case for both for instances that use Xen and C5 that uses the new KVM-based hypervisor."
So that seems to confirm an offload to hardware for storage.
1) You don't have to pass PCIe through to hardware to get performance that makes it look like you have.
2) We know they're offloading though, so here's what I think it looks like.
It seems more likely that they've put in a software device model of NVME as a replacement for the BlkBack software device model that the BlkFront driver talked to. Not much different than the software e1000 NIC that xen/qemu already supports.
That said, with their virtualizable Annapurna wonder-NIC, they could be doing it in "hardware", though even in that case a reasonable part of the device model would be software, just running on a NPU, not a CPU.
Hopefully Amazon will disclose more details.