also interested in the upper limit of a micro vm, like how big can it get? 64gb memory? not really micro any more and maybe a traditional VM would be a better fit.
I have to imagine that Fargate on Windows doesn't use Firecracker though, right? Firecracker needs kernel level changes to work properly, and the open source version doesn't let you run anything but Linux.
I have no idea how it works under the hood. Knowing what I know about Firecracker from watching the publicly available videos, I was shocked and thought it would never happen.
On the other hand, CodeBuild has supported Windows containers for years and at least CodeBuild for Linux is based on Fargate, so the service team figured something out. (I had to figure out how to word that. I can’t say “they figured it out” since I work for the same company. But I couldn’t say “we” since I’m so far removed from any service team in the consulting department that it would be disingenuous)
The biggest VM we've been running for dev environments have 12GB RAM, 8vCPUs and 30GB disk. I've also done some tests with 16GB RAM and that worked well too. Have yet to find an upper limit.
Another (unrelated) test we've done is on overprovisioning memory. We were able to run 200 VMs (all running Vite dev server where a file was changed every second) with 2GB RAM per VM, on a node with 128GB RAM. Because we were mapping the memory files on disk directly to the VM, the VM would automatically "swap" the memory back to the memory file when it had memory pressure. The bottleneck here was CPU.
The "micro" in microvms is less about size and more about resources. A typical virtual machine under Xen or KVM (para)virtualizes a lot of hardware and emulates a lot of devices, so that the operating system sees it as a normal machine.
The microVM emulates the minimal possible set of devices needed to run, such as disks and network devices, and in the specific case of firecracker, through the use of the virtio model. So it can theoretically use huge amounts of memory of a large vCPU count and still be a microvm.
Unrelated TIL: AWS Fargate has supported Windows since last October. I work at AWS and “specialize” in serverless and I didn’t know that.