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README: mention kexec tarballs

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Jörg Thalheim 2022-09-05 08:58:49 +02:00
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# nixos-images
Automatically build images for NixOS. This project is meant to extend images builded by hydra.nixos.org.
We currently build:
Automatically weekly updated images for NixOS. This project is intended to extend the images created by hydra.nixos.org.
We are currently creating the images listed below:
- netboot images: see [bzImage](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/bzImage-x86_64-linux) and [initrd](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/bzImage-x86_64-linux)
- and [kexec](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/kexec-bundle-x86_64-linux) images
## Netboot images
Images are weekly updated.
You can boot the netboot image using this [ipxe script](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/netboot-x86_64-linux.ipxe).
It consists of the [kernel image](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/bzImage-x86_64-linux) and [initrd](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/bzImage-x86_64-linux).
## Kexec tarballs
Kexec is a mechanism in Linux to load a new kernel from a running Linux to
replace the current kernel. This is useful for booting the Nixos installer from
existing Linux distributions, such as server provider that do not offer a NixOS
option. After running kexec, the NixOS exists only in memory. At the time of
writing, this requires at least 2.5GB of RAM in the system. If not enough RAM
is available, the initrd cannot be loaded. Because the NixOS runs only in RAM,
users can reformat all the system's discs to prepare for a new NixOS
installation.
Currently, there are two variants of kexec: [nixos-kexec-installer](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/nixos-kexec-installer-x86_64-linux.tar.xz)
and [kexec bundle](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/kexec-bundle-x86_64-linux).
The nixos-kexec-installer tarball is the new preferred method.
It can be booted as follows:
```
curl -sL https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/nixos-kexec-installer-x86_64-linux.tar.xz | tar -xf -C /root
/root/kexec/run
```
The script copies existing sshd host keys and ssh keys from
`/root/.ssh/authorized_keys` and `/etc/ssh/authorized_keys.d/root` to the booted nixos machine. The actual kexec happens with a slight delay (6s).
This allows for easier integration into automated nixos installation scripts, since you can cleanly disconnect from the running machine before the kexec takes place.
The tarball is also designed to be run from NixOS, which can be useful for new installations
We also have [kexec-bundle](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/kexec-bundle-x86_64-linux),
which is a self-extracting archive from [nixos-generators](https://github.com/nix-community/nixos-generators). This version unpacks itself to `/` and possibly overlays the existing `/nix/store` with its own files. with its own files.