Firecracker design figures → FluxVM (general control plane)

FluxVM is a host-local VM control plane (POSITIONING.md), not a disposable-only product. Firecracker’s design figures still matter — but as layered isolation and optional density targets, not as the product category.

Sources: design.md, SPECIFICATION.md, prod-host-setup.md.

Two adoption tracks

flowchart LR
  subgraph trackA [Track A Production isolation]
    KVM[KVM barrier]
    RL[Virtio rate limiters]
    Jail[Jailer plus seccomp]
    CG[cgroups]
    Egress[Host egress Fabric or none]
  end
  subgraph trackB [Track B MicroVM density optional]
    Overhead["VMM RSS target"]
    Boot["Boot to ready"]
    Mut["Mutation per core"]
  end
  FC[Firecracker figures] --> trackA
  FC --> trackB
  trackA --> AllBackends[All FluxVM backends where applicable]
  trackB --> FcPath[Firecracker and flux-vm sandbox path]

Track	Who cares	Adopt?
A — Production isolation	Every production host (long-lived or short-lived guests)	Yes — default hardening
B — MicroVM density	Sandbox / dense Firecracker packing / optional disposable use	Yes — measure & publish; not product identity

Disposable / TTL workloads are a use pattern on top of Track A (+ Track B when dense). Longer-lived guests use the same Track A barriers without claiming FC mutation rates.

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Track A — Production isolation (adopt for general CP)

Maps Firecracker’s host-integration and threat-containment figures onto FluxVM:

Layer	Firecracker	FluxVM
KVM	Hardware virtualization boundary	All backends
Virtio I/O rate limiters	Token bucket at net/blk	Create fields net_mbit_limit / net_pps_limit / blk_mbit_limit / blk_ops_limit → Firecracker JSON and FluxVm/fluxvm-hypervisor boot config
Jailer + seccomp	Production-only jailer; auto seccomp	[jailer] enabled + enforce; stock FC seccomp; in-tree seccomp.rs for fluxvm_engine=kvm
Host egress filter	Required outside VMM	Network Fabric GA or network.mode=none
Cgroups	Per-microVM quota/cpuset	fluxvm-cgroup on every VM
Serial / logs	Disable 8250 in prod; bound stdout	Jailed FC default boot args; journald/logrotate checklist

Config profile: configs/production-hardening.toml (merge with your listen/auth/dataplane settings). Checklist: PRODUCTION.md.

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Track B — MicroVM density (optional; Firecracker SoT)

Lab measurement targets only — not FluxVM SLAs and not the product pitch. Engine SoT: Firecracker (fluxvm_engine unset / firecracker). In-tree KVM is lab comparison (kvm-density.md).

Figure	Firecracker SPEC / design	FluxVM measurement
VMM RSS overhead	≤ 5 MiB (1 vCPU / 128 MiB)	bench-sandbox.sh + REPORT_RSS=1
Boot to guest ready	≤ 125 ms InstanceStart → init	boot_to_ready_ms (includes control plane)
Mutation rate	5 microVMs / host-core / sec	bench-density.sh → mutation_per_core_sec
CPU oversubscription	Operator-controlled	cgroup v2 + warm pools + [policy] default_cpu_quota_percent / memory_max_equals_guest — oversubscription.md
IO Gbps / GiB/s	FC SPEC	Not claimed

BENCH_N=5 REPORT_RSS=1 MEMORY_MIB=128 IMAGE=… KERNEL=… ./scripts/bench-sandbox.sh
BENCH_N=8 DENSITY_MEMORY_MIB=128 ./scripts/bench-density.sh

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Adopt / skip / next (post-repositioning)

Item	Decision
Threat-containment layers (Track A)	Adopted — jailer enforce, rate limiters, egress checklist, serial-off when jailed
Rate limiters on plain FC + FluxVm hypervisor path	Adopted — request fields on both
Density figures (Track B)	Adopted as optional benches — do not lead marketing with them
MMDS	Skip — vsock guest-agent + seed/cloud-init
CPU templates	Adopted (static) — cpu_template on Firecracker + FluxVm firecracker-engine; rejected elsewhere
FC Gbps claims	Defer — need pinned emulation cores
Plain Firecracker + FluxVm /v1/vms snapshots	Adopted — FC /snapshot/*; FluxVm control SnapshotSave/Restore; CH/QEMU unchanged
Explicit oversubscription knobs	Adopted — policy defaults + oversubscription.md
Replace QEMU Secure Containers with FC device model	Skip — SC stays QEMU SoT

Ranked follow-ups historically FC1–FC3 (now done): NEXT-FEATURES.md.

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Messaging (aligned with POSITIONING)

Say: “FluxVM adopts Firecracker’s isolation layering for production hosts; density figures are optional for the microVM/sandbox path.”

Avoid: Framing FluxVM as disposable-only because FC is serverless-oriented; citing unpublished Track B numbers as product guarantees.