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Comment by dwroberts | Hacker News Reader

> I see firecracker replacing docker on the horizon

I don’t think this is going to happen because they serve different purposes. Having to boot an entire OS inside a VM is a step backwards compared to containerisation. There are definitely use cases for isolating containers by running them inside a VM (see: Kata) but it generally ‘replacing’ docker, I don’t think that is on the horizon at all

(Also: you need KVM available, you need a rootfs to boot which will be larger than a container, it has no built in support for mounts or really any kind of communication with the host unless you explicitly set up networking etc for it)

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> Having to boot an entire OS inside a VM

The whole point of Firecracker over normal VM solutions is that "booting an OS" takes milliseconds.

Still, I don't think Docker is at risk of being replaced just yet because of the resource management benefits you get from sharing a kernel.

Balloon devices go a long way for ram sharing, though not all the way. CPU scheduling is great.

There's definitely a tradeoff, but IME it's not as drastic as you first think.

Whether it takes milliseconds or seconds though, you are still doing it. You’re still just running a VM

> The whole point of Firecracker over normal VM solutions is that "booting an OS" takes milliseconds.

Define "OS". A barebones OS will indeed start in microseconds. But then your app/service won't. Because it will likely need a gazillion things that a barebones OS doesn't provide. Suddenly the startup time isn't microseconds or even milliseconds.

Sadly, we never got the promise of unikernels, and everything requires the full-blown OS to run anything

The VM's I'm talking about don't run some kind of weird bespoke OS, just a normal Linux kernel. You can make Linux boot quite quickly if you strip out everything but the hardware you virtualise, and if you resume a snapshot from CoW RAM, you don't even need to do that.

I don't have any recent measurements myself, but the startup time for a full Linux kernel using Firecracker's snapshotting feature seems to be around 30ms if you optimise calls for latency: https://dev.to/adwitiya/how-i-built-sandboxes-that-boot-in-2...

The measurements in that blog post place Firecracker above Docker in terms of startup performance. Most of that delay is probably the Docker daemon rather than Linux namespace APIs, but that's beside the point.

Huggingface places Firecracker startup time between 100-300ms while Docker is at 50-200ms, based on Kata: https://huggingface.co/blog/agentbox-master/firecracker-vs-d...

Either way, latency is in the same ballpark of "low enough that it only matters in edge cases".

I would say that kind of got halfway there, when doing serverless.

The gazillion things that the OS doesn't provide is taken care by language runtimes, which can perfectly run directly on top of type 1 hypervisors.

In similar way how some of those languages have bare metal implementations for embedded development, where the runtime takes the OS role.

Naturally on languages with thin runtimes and heavy reliance on POSIX like C and C++, this isn't as straightforward.

Hmm... "resource management benefits you get from sharing a kernel" costs the security risks you incur from not having kernel-level isolation.

These days I'm looking at microvms (esp. smolvm from smolmachines.com) by default, for anything where I'd previously have reached for docker (via colima or more recently orbstack).

> Having to boot an entire OS inside a VM is a step backwards compared to containerisation

If it boots faster than a container ...

Also containers don't work well with remote filesystems, or filesystems in general, because the admin always limits you.

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