notes: initrd.txt: add information about initrd vs initramfs

There are some big differences between initrd and initramfs. This
text document lists them and the advantages/disadvantages.

Signed-off-by: John Ogness <john.ogness@linutronix.de>

1 files changed, 41 insertions, 0 deletions

diff --git a/schulung_tools/notes/initrd.txt b/schulung_tools/notes/initrd.txt
new file mode 100644
index 0000000..e88b20b
--- /dev/null
+++ b/schulung_tools/notes/initrd.txt
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+#
+# For both initrd variants it is assumed the root filesystem files are
+# located at /home/devel/rootfs ...
+#
+
+
+# To create the archive-based initial ramdisk:
+
+cd /home/devel/rootfs
+find . | cpio -o -H newc | gzip -9c > ../initrd.cpio.gz
+
+
+# To create the image-based initial ramdisk:
+
+dd if=/dev/zero of=initrd.img bs=4M count=1
+/sbin/mkfs.ext2 initrd.img
+sudo mount -o loop initrd.img /mnt
+sudo cp -a /home/devel/rootfs/. /mnt/
+sudo umount /mnt
+gzip -9 initrd.img
+
+
+#
+# The image-based initial ramdisk is the old way of doing things. It
+# really has no advantages. But it does have disadvantages:
+#
+#   - the size if limited by the image
+#   - the size is limited by the kernel configurations
+#     (CONFIG_BLK_DEV_RAM, CONFIG_BLK_DEV_RAM_SIZE)
+#   - the kernel root= parameter must be set to /dev/ram0
+#   - the real root filesystem must unmount it after pivot_root
+#     (which means the real root filesystem has knowledge of an initrd)
+#
+# The archive-based initial ramdisk has the following differences/advantages:
+#
+#   - very simple to create (no root rights required!)
+#   - uses exactly as much RAM as needed (grows dynamically)
+#   - does not require any special boot arguments
+#   - does not need to be "cleaned up" after switch_root
+#     (the real root filesystem has no knowledge that an initrd existed)
+#