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Grendel, the Bare-Metal Provisioner

Grendel is a solution used to deploy and manage HPC (High Performance Computing) clusters. It is designed to automate the process of configuring and installing software on compute nodes using a custom DHCP, DNS, TFTP, and HTTP server written in Go.


The architecture is the following:


Grendel is deployed as a container and runs multiple network services responsible for the provisioning of bare-metal systems.

The Service Processor (SP) controls the hardware and is used to perform out-of-band hardware control (e.g. Integrated Management Module (IMM), Flexible Service Processor (FSP), Baseboard Management Controller (BMC), etc).

The Service Processor is connected to Grendel via the Service Network.

The Management Network is used for OS provisioning (via PXE).

Why Grendel ?

Our main criteria for choosing a bare metal provisioning solution is:

  • Operating system image management: provisioning methods, supported operating systems, ease of use.
  • BMC configuration (IPMI, ...)
  • Configuration management (declarative, post-boot scripts, backups, ...)

While a lot of solutions exist for bare-metal provisioning like OpenStack Ironic or MAAS, only a few can do disk-less provisioning.

Disk-less (or Stateless) provisioning is based on an OverlayFS root, with the OS being loaded from a SquashFS image. The OverlayFS is mounted as a tmpfs, that is, in the RAM.


Since the root is mounted in RAM, restarting a node will "clean it up", similar to a Docker container.

With OverlayFS, we follow a proper DevOps practice where the architecture is immutable and mutable data is stored on disks.

How does it work ?

OS Image Building

Grendel doesn't have an OS image builder. This is because we do not want the users to use a non-standard tool to build their OS images.

Instead, we prefer to use Packer, mksquashfs and Dracut to automate the building of OS images, linux kernels and linux initramfs.

The steps to build disklessn OS image are the following:

  1. Use Packer to build an image that includes the operating system and any additional packages or customizations you require. This image will serve as the base for the diskless OS.
  2. Mount the OS image, chroot in the OS image and execute dracut to generate initramfs.
  3. Extract the kernel and initramfs
  4. Use mksquashfs to create a squashfs filesystem image that will serve as the base layer of the overlayfs. The squashfs image should include any read-only files that will be shared across multiple diskless nodes.


The steps to provision an OS image are the following:

  1. Enable network boot on the BIOS of the nodes. This will cause the node to broadcast a DHCP request for an IP address and PXE server.
  2. The Grendel DHCP server will respond with an IP address based on the node's MAC address, and send the Network Boot Program (NBP) - in this case, the iPXE firmware.
  3. The iPXE firmware will then download the Linux kernel and initramfs.
  4. The Linux kernel options* will be used to download the squashfs file from the Grendel server and mount the OS image as a /dev/loop device.
  5. Once the base image is mounted, an overlayfs will be created using the loop device as the base image and an upper layer created using a tmpfs (RAM). This will provide a writeable layer for temporary files and system changes.


After initializing the operating system image, a SystemD service can be utilized to retrieve the postscript file stored on the Grendel HTTP server and execute it.