Merge pull request #14 from nix-community/kexec-tarballs

README: mention kexec tarballs

README.md

@@ -1,9 +1,40 @@
 # nixos-images
 
-Automatically build images for NixOS. This project is meant to extend images builded by hydra.nixos.org.
+Automatically weekly updated images for NixOS. This project is intended to extend the images created by hydra.nixos.org.
-We currently build:
+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)
+## Netboot images
-- and [kexec](https://github.com/nix-community/nixos-images/releases/download/nixos-unstable/kexec-bundle-x86_64-linux) 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.