Add initramfs write extension

This creates a gzipped cpio archive that may be used as an initramfs.
It is hard-coded to use gzip to compress the initramfs, since it's the
most common way to do it.

This is unfortunate, since the busybox gzip utility only allows maximum
compression, which is rather slow and doesn't give progress reporting,
so you can easily think it's gotten stuck.

It's possible to use other compression formats, but they need the kernel
to be built with them supported, and in the case of lz4, unusual userland
tools to create it, since the version of lz4 supported in the kernel is
not what the standard lz4 tools produce.

1 files changed, 35 insertions, 0 deletions

diff --git a/morphlib/exts/initramfs.write.help b/morphlib/exts/initramfs.write.help
new file mode 100644
index 00000000..29a9d266
--- /dev/null
+++ b/morphlib/exts/initramfs.write.help
@@ -0,0 +1,35 @@
+help: |
+    Create an initramfs for a system by taking an existing system and
+    converting it to the appropriate format.
+
+    The system must have a `/init` executable as the userland entry-point.
+    This can have a different path, if `rdinit=$path` is added to
+    the kernel command line. This can be added to the `rawdisk`,
+    `virtualbox-ssh` and `kvm` write extensions with the `KERNEL_CMDLINE`
+    option.
+
+    It is possible to use a ramfs as the final rootfs without a `/init`
+    executable, by setting `root=/dev/mem`, or `rdinit=/sbin/init`,
+    but this is beyond the scope for the `initramfs.write` extension.
+
+    The intended use of initramfs.write is to be part of a nested
+    deployment, so the parent system has an initramfs stored as
+    `/boot/initramfs.gz`. See the following example:
+
+        name: initramfs-test
+        kind: cluster
+        systems:
+        - morph: minimal-system-x86_64-generic
+          deploy:
+            system:
+              type: rawdisk
+              location: initramfs-system-x86_64.img
+              DISK_SIZE: 1G
+              HOSTNAME: initramfs-system
+              INITRAMFS_PATH: boot/initramfs.gz
+          subsystems:
+          - morph: initramfs-x86_64
+            deploy:
+              initramfs:
+                type: initramfs
+                location: boot/initramfs.gz