cloudparts

Helios

Oxide’s distribution of illumos, the operating system on every sled’s host CPU in the Oxide rack, where it runs VMs on its bhyve hypervisor and the control plane in zones.

What it is
Hypervisors
Used for
Virtual machines
Type
Open source
Made by
Oxide Computer Company
License
MPL 2.0 (Helios build tooling); the illumos core is mostly CDDL
Facts checked
13 October 2026
Website Source code

What it is

Helios is the host operating system of the Oxide rack: the illumos kernel and userland, built from Oxide’s stlouis branch of illumos-gate. Its README calls the packages “mostly stock illumos” with additions for Oxide hardware. The Helios repository holds the tools that build it from several consolidations: the OS itself, build scripts for packages beyond the core OS, a zone brand for Omicron components, and boot tools.

On a sled it boots without a UEFI BIOS. The AMD platform security processor loads Oxide’s tiny phbl loader from flash, which switches the CPU into 64-bit mode, unpacks a compressed boot archive and jumps to the illumos kernel. The OS that Omicron installs on a sled is a whole image: a 32 MiB boot ROM plus a root file system ramdisk.

Once up, Helios provides what the rest of the rack is built on: bhyve as the kernel half of the hypervisor (Propolis is the user-space half), zones to isolate each control plane service and each VM’s monitor, ZFS on the sleds’ SSDs, SMF to supervise services, the Fault Management Architecture, and DTrace and core files for debugging in the field. OPTE’s virtual networking runs inside it as a kernel module.

Why teams pick it

  • Isolation primitives built in: each service and each VM’s Propolis runs in its own zone with its own network stack
  • Debugging tools Oxide wanted in production: DTrace for live tracing, rich core files for postmortem analysis and FMA for hardware faults
  • A small, controlled boot path with no UEFI firmware stack, and the OS delivered as one image
  • Open source, including the kernel branch and the build tooling, so customers can read the OS their rack runs

Watch out for

  • It is built for Oxide hardware and x86 only; on a laptop it runs as a VM for development, not on Arm Macs
  • Not every consolidation is public: AMD CPU firmware, Chelsio T6 NIC firmware and some low-level tools are listed as “will be available in future”
  • illumos has a much smaller community than Linux, so Oxide takes on more of the maintenance itself
  • Customers never run it directly: it is the rack’s host OS, and guests bring their own operating systems

Alternatives

What all hypervisors have in common →Compare side by side →
ACRNDormantOpen sourceA small type-1 hypervisor for Intel-based embedded and industrial systems that runs real-time and safety VMs on dedicated cores next to Linux, Windows or Android guests on the same board.Cloud HypervisorOpen sourceA Rust virtual machine monitor for modern cloud VMs on KVM or Microsoft’s hypervisor: virtio devices only, plus hotplug, VFIO passthrough, live migration, snapshots and Windows guests.KVM (Kernel-based Virtual Machine)Open sourceThe hypervisor built into the Linux kernel: it runs guest code directly on the CPU through /dev/kvm, and leaves device emulation to a user-space program such as QEMU or Firecracker.libvirtOpen sourceA stable API, daemon and virsh command line for managing virtual machines, storage and networks, with drivers for QEMU/KVM, Xen, LXC, Cloud Hypervisor, VMware ESX, Hyper-V and more.Microsoft Hyper-VCommercialMicrosoft’s type-1 hypervisor, built into Windows Server and Windows 10 and 11 Pro and Enterprise: a thin hypervisor under a Windows root partition, which also underpins Azure’s hypervisor and Windows features like VBS, WSL 2 and Windows Sandbox.Oracle VirtualBoxOpen sourceOracle’s free, open-source desktop hypervisor: a type-2 hypervisor that runs VMs as an application on Windows, macOS, Linux and Solaris, scriptable with VBoxManage and the default provider for Vagrant.PropolisOpen sourceOxide’s virtual machine monitor: a Rust program that pairs with the bhyve hypervisor in the illumos kernel and emulates the devices each VM in an Oxide rack sees.Proxmox Virtual EnvironmentOpen sourceA Debian-based server platform from Vienna that runs KVM virtual machines and LXC containers, with clustering, high availability, Ceph and ZFS storage, backups and a web interface in one free ISO.QEMUOpen sourceThe open-source machine emulator and virtualizer: it emulates whole computers in software, or runs them at native speed on a hypervisor such as KVM, and supplies the virtual devices either way.VMware ESXiCommercialVMware’s bare-metal hypervisor (called ESX again since VCF 9), whose own kernel runs VMs directly on servers while vCenter adds vMotion, HA, DRS and vSAN; since Broadcom bought VMware in 2023 it is sold only as per-core subscriptions.Xen Project hypervisorOpen sourceThe open-source type-1 hypervisor from Cambridge that ran the first Amazon EC2 instances: a small microkernel-style core that boots first, with drivers and management in a privileged VM called dom0.

How it fits with other technologies

Build illumos the Helios way and install it as a new boot environment (on a Helios machine, from the README) · bash

git clone https://github.com/oxidecomputer/helios.git
cd helios
gmake setup                         # builds ./helios-build and clones illumos (stlouis)
./helios-build build-illumos -q     # a quick, non-release build
tail -F projects/illumos/log/nightly.log
./helios-build onu -t my-be-name    # install into boot environment my-be-name
reboot

See it in a real stack

No architecture uses Helios yet. Drop it into the playground and wire it up yourself.Open playground →
Where it came from2020–2024

The Oxide rack’s host CPU had to run customer VMs, distributed storage and the control plane, so the choice of operating system and hypervisor came together. RFD 26, by Joshua M. Clulow, Patrick Mooney and Robert Mustacchi, compared KVM on Linux with bhyve on illumos. Beyond the hypervisor, it weighed what a small team supporting critical infrastructure would need: service supervision, isolation, fault management, and the ability to debug a defect fully from its first occurrence, with DTrace and postmortem core files. It also noted that illumos raised no objection to kernel modules written outside the core tree or in Rust.

By June 2021, after reducing the risk of building on illumos and working with the Rust community on illumos toolchain issues, Oxide decided on Helios, its illumos distribution, as the host OS, with bhyve and Propolis for guests. Its May 2021 milestone demo had already run Helios on an AMD EPYC 7402P system with Omicron, CockroachDB and ClickHouse on top. The same reasoning led Oxide to drop the UEFI stack in favor of its own boot path.

Helios shipped in the first racks in 2023 and was open-sourced in January 2024, prompting an Oxide and Friends episode on its history. Its packages remain mostly stock illumos.

Why it’s built this way

illumos rather than Linux
RFD 26 valued illumos’s zones, SMF, FMA, DTrace and postmortem debugging for a small team that must fix defects from few incidents, and its stable kernel interfaces and openness to modules in Rust.
No UEFI firmware on the host
Oxide intended to discard the UEFI stack for host bootstrap in favor of something that met its needs more directly; phbl loads the OS straight from flash.

Timeline

  1. 2020

    Repository started (August)

  2. 2021

    Runs on an AMD EPYC 7402P at Oxide’s May milestone demo; RFD 26 picks Helios as the host OS (June)

  3. 2023

    Ships as the host OS of the first Oxide racks

  4. 2024

    Open-sourced (January)

Sources8 · facts checked 13 October 2026

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