Making A Flexible Initial Ramdisk ================================= Jayson Vantuyl This example of booting from AoE storage was contributed via email by Jayson Vantuyl and edited by Ed L. Cashin. Introduction ------------ I don't have anything booting off of PXE, I usually flash etherboot directly into the flash ROM. Getting pxeboot (PXE version of etherboot) is fairly easy and I'm sure you guys can do that from the http://www.tldp.org/HOWTO/Network-boot-HOWTO/[Network Boot and Exotic Root HOWTO]. I also include a link to PXELINUX, the PXE port of the SYSLINUX boot loader, <>. After that, it gets a little interesting (defined "complicated"). The way I do it for my clients involves building a Gentoo system in a // http://en.wikipedia.org/wiki/Chroot[chroot] chroot and using it to build a stripped down boot environment that can be loaded either as an initrd or using initramfs. I highly recommend this approach. It's the only way I know of, short of using a Linux From Scratch approach, to make a stripped down boot environment. Following is an EXTREME crash course in doing this. Once this is done, it is very easy to keep up to date. It also has the advantage that it can be configured to make binary packages that can be used to update any test chroot builds you have. Note that the following instructions will create a decent preboot environment for setting up an AoE device for use as a root filesystem. However, it is fairly easy to add software RAID and LVM support. The System Core --------------- Getting the System Core ~~~~~~~~~~~~~~~~~~~~~~~ You must first download the Gentoo embedded stage3 from the web. Any mirror will work. An example URL: http://gentoo.osuosl.org/experimental/x86/embedded/stages/stage3-x86-uclibc-2005.0.tar.bz2[] Note that this is a 35 megabyte build of a Gentoo embedded system. Make a directory, uncompress it. All further commands are executed in this chroot. It is extremely likely I've left something out, as this is initially quite a process. Note that it is not necessary to do this on a Gentoo Linux system. I originally did this on a Debian Sarge system (and have done it on Redhat as well). Modifying the System Core ~~~~~~~~~~~~~~~~~~~~~~~~~ It may be necessary to modify files in the etc directory. The `etc/make.conf` file can be tweaked to create packages that can later be utilized to build alternate chroots without rebuilding the packages from source (which Gentoo does by default). It is also where you specify USE flags (which allow various build options to be tweaked). You may also need to create `etc/portage/packages.unmask` and `etc/portage/packages.keywords` in order to open up the packages you want. I only modify the `make.conf` file. In my build system, I have the following /etc/make.conf: CFLAGS="-Os -pipe" CHOST="i386-pc-linux-uclibc" CXXFLAGS="-Os -pipe" FEATURES="buildpkg" From here you should update Portage (the packaging system for Gentoo). This can be done by issuing: emerge sync Packages can be updated by issueing: emerge -u world Extending the System Core ~~~~~~~~~~~~~~~~~~~~~~~~~ From here, you can make a chroot for use as an initrd roughly like this: mkdir -p /images/aoe ROOT=/images/aoe emerge busybox mdadm This builds a base system with nothing but mdadm and busybox. This is all I've needed to start a Software RAID and mount it as root. Note that you may not need the mdadm, as you have RAID already available for you. You might wish to try to build LVM2, though, since it IS extremely useful for your customers. Similarly, CLVM might be an option too. This system is based on uclibc to make it small, so these LVM options may not respond well to that. Note that klibc is also an option, but not available this way. At this point I usually make links in `/bin` as such: ln -s busybox sh ln -s busybox ls ln -s busybox echo ln -s busybox sleep And links in `/sbin` as such: ln -s ../bin/busybox ifconfig ln -s ../bin/busybox ip ln -s ../bin/busybox modprobe ln -s ../bin/busybox mount This system needs to have a `/linuxrc` file that provides instructions for when it is used as an initrd (not sure how this works with initramfs). I create one with the following contents: #!/bin/sh # Load Config . /config # Load Modules for MOD in $MODULES ; do /sbin/modprobe $MOD done # Bring Up Interfaces for IFACE in $AOE_IFACES ; do /sbin/ip link set up dev $IFACE done # Settle For A Moment (sometimes link takes a sec to come up) /bin/sleep 4 # Discover AOE /bin/echo 1 > /dev/aoe/discover # Wait for AOE to Figure Itself Out /bin/sleep 2 # Start Any Raids /sbin/mdadm -A -s # Upon Exit, Should find the new root, mount it, and be happy... Note that the above file doesn't change per unit, but the config file it references does. Also, I think it could be optimized a little, as busybox will execute its own internal commands without the symlinks. I may refine this further. If I do, I'll send you an update. I create a config file that is customized as follows: MODULES=e1000 # Or whatever modules you need AOE_IFACES=eth1 # Or whatever interfaces are appropriate I also create an /etc/mdadm.conf to locate the RAID. Something like this: DEVICE /dev/aoe/e[0-9].[0-9] DEVICE /dev/aoe/e[0-9].[0-9]p[1-9] DEVICE /dev/aoe/e[0-9].[0-9]p1[0-5] ARRAY /dev/md0 uuid=01234567:89ABCDEF:01234567:89ABCDEF Creating an Initrd ~~~~~~~~~~~~~~~~~~ You can then run mkcramfs on the directory to make an approximately 2MB initrd. When you boot the kernel, make sure to load the initrd and to put on the command line `root=/dev/md0` or whatever is appropriate. The great thing about using an initrd this way is that your linuxrc only has to set up the environment, but it still uses the "root=" option to find the root. This plays well with most bootloader set ups (which expect to modify root= instead of tweaking the initrd like some setups do) and also preserves the ability to use `init=/bin/sh` for disaster recovery or major maintenance. Boot Overview ------------- To clarify, the early boot process of the 2.6 kernel (and 2.4 is similar) goes something like this: - boot - create a magic ramfs used by the preboot code (possibly containing initramfs stuff which I don't yet understand) - uncompress an initrd if one is detected in /initrd - chroot to the /initrd and execute /linuxrc -- NOTE: This is where most initrd maintainers mess it all up. They play silly pivot_root tricks to manually mount their own root and make it the root fs--ignoring any init= and root= options. Just let the linuxrc script terminate and the kernel picks up where it left off. - Using the newly prepared system, mount the root specified in `root=...` - If /initrd exists on the new root, move the /initrd mount there - chroot to that root and execute the init as specified by `init=...` If you let the kernel do all of the work, you can see that your linuxrc file need only configure whatever is necessary for your magic. It could even load the config file from the network. If you specify `ip=dhcp` on the kernel command line and build your kernel with IP Autoconfiguration DHCP support, you can even have the interface already configured via DHCP and retrieve the configuration into a ramfs mounted `/tmp` using busybox's `wget` command. Conclusion ---------- This is a ridiculously powerful preboot environment considering it can be obtained with less than 5MB of kernel and initrd. I am strongly convinced that this is evidence of some extreme mojo in the design of the Linux kernel. These guys really have a plan for preboot and it exists specifically to accommodate more complex boot devices just like an AoE system from Coraid. Also, I don't usually remove unnecessary files from the chroot because I'm, well, lazy. If you remove them, then it gets down to 1.3MB. This should be small enough that it (along with the kernel) will fit on most flash ROMs. Should you be designing a product, you may even be able to use a bootloader like ROLO to flash it all directly into the BIOS ROM. Exciting! [[pxelinux]] At this point you'll need to boot of the network. You might try the pxelinux bootloader. A pretty comprehensive description is available here: http://syslinux.zytor.com/pxe.php[] It doesn't take much more than the right TFTP server and some DHCP magic. I know that the appropriate versions of software are available in the Gentoo distribution. I *THINK* Debian Sarge has it all too (although you may need to build a different TFTP server). Make no mistake, the above is probably the ultimate system for building and embedded Linux system available right now. You get the ability to have a stripped down system but still have the flexibility required to have binary packages that can be used for speedy and flexible system image generation. Appendix A: Copyright --------------------- This document is licensed under the Creative Commons Attribution License. It's basically Public Domain provided that you credit me with having written it and don't sue me for providing it. The text of the license is here: http://creativecommons.org/licenses/by/2.5/legalcode[] .......................................................................... Creative Commons Attribution License THE WORK (AS DEFINED BELOW) IS PROVIDED UNDER THE TERMS OF THIS CREATIVE COMMONS PUBLIC LICENSE ("CCPL" OR "LICENSE"). THE WORK IS PROTECTED BY COPYRIGHT AND/OR OTHER APPLICABLE LAW. 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This License may not be modified without the mutual written agreement of the Licensor and You. .......................................................................... Appendix B: Contributing ------------------------ The asciidoc-format source to this document is available link:aoeboot.txt[here], and corrections or updates are welcome in the form of a unified diff. Be sure to include text for the ChangeLog. Appendix C: ChangeLog --------------------- // The ChangeLog entry format is: // minus character, date, name, email, newline // plus character, newline // description paragraph, newline // newline - 20060103 Ed L. Cashin + Initial publication based on Jayson Vantuyl's original email text and subsequent preliminary changes.