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XenDemoCD 1.3
University of Cambridge Computer Laboratory
24 Jan 2004
http://www.cl.cam.ac.uk/netos/xen
Welcome to the Xen Demo CD!
Executive Summary
=================
This CD is a standalone demo of the Xen Virtual Machine Monitor (VMM)
and Linux-2.4 OS port (Xenlinux). It runs entirely off the CD,
without requiring hard disk installation. This is achieved using a RAM
disk to store mutable file system data while using the CD for
everything else. The CD can also be used for installing Xen/Xenlinux
to disk, and includes a source code snapshot along with all of the
tools required to build it.
Booting the CD
==============
It should be possible to get Xen working with any relatively modern
hardware supported by standard Linux. However, the version of XenLinux
built for the DemoCD is fairly h/w specific. If you need other
hardware, you'll have to configure and build your own xenlinux kernel.
Xen does require an 'i686'-class CPU or newer, so won't work on 486's
or plain Pentiums.
We have compiled in drivers for the following hardware:
CPU: Pentium Pro/II/III/IV/Xeon, Athlon (i.e. P6 or newer) SMP supported
IDE: Intel PIIX chipset, others will be PIO only (slow)
SCSI: Adaptec / Dell PERC Raid (aacraid), fusion MPT, megaraid, Adaptec aic7xxx
Net: Recommended: Intel e1000, Broadcom BCM57xx (tg3), 3c905 (3c59x)
Also supported: pcnet32, Intel e100, tulip
Because of the demo CD's use of RAM disks, make sure you have plenty
of RAM (256MB+).
To try out the Demo, boot from CD (you may need to change your BIOS
configuration to do this), then select one of the four boot options
from the Grub menu:
Xen / linux-2.4.26
Xen / linux-2.4.26 using cmdline IP configuration
Xen / linux-2.4.26 in "safe mode"
linux-2.4.22
The last option is a plain linux kernel that runs on the bare machine,
and is included simply to help diagnose driver compatibility
problems. The "safe mode" boot option might be useful if you're having
problems getting Xen to work with your hardware, as it disables various
features such as SMP, and enables some debugging.
If you are going for a command line IP config, hit "e" at
the grub menu, then edit the "ip=" parameters to reflect your setup
e.g. "ip=<ipaddr>::<gateway>:<netmask>::eth0:off". It shouldn't be
necessary to set either the nfs server or hostname
parameters. Alternatively, once Xenlinux has booted you can login and
setup networking with 'dhclient' or 'ifconfig' and 'route' in the
normal way.
To make things easier for yourself, it's worth trying to arrange for an
IP address which is the first in a sequential range of free IP
addresses. It's useful to give each VM instance its own public IP
address (though it is possible to do NAT or use private addresses),
and the configuration files on the CD allocate IP addresses
sequentially for subsequent domains unless told otherwise.
After selecting the kernel to boot, stand back and watch Xen boot,
closely followed by "domain 0" running the Xenlinux kernel. The boot
messages can also sent to the serial line by specifying the baud rate
on the Xen cmdline (e.g., 'com1=9600,8n1'); this can be very useful
for debugging should anything important scroll off the screen. Xen's
startup messages will look quite familiar as much of the hardware
initialisation (SMP boot, apic setup) and device drivers are derived
from Linux.
If everything is well, you should see the linux rc scripts start a
bunch of standard services including sshd. Login on the console or
via ssh::
username: user root
password: xendemo xendemo
Once logged in, it should look just like any regular linux box. All
the usual tools and commands should work as per usual. However,
because of the poor random access performance of CD drives, the
machine will feel very slugish, and you may run out of memory if you
make significant modifications to the ramfs filesystem -- for the full
experience, install a Xen and Xenlinux image on you hard drive :-)
You can configure networking, either with 'dhclient' or manually via
'ifconfig' and 'route', remembering to edit /etc/resolv.conf if you
want DNS to work.
You can start an X server with 'startx'. It defaults to a conservative
1024x768, but you can edit the script for higher resoloutions. The CD
contains a load of standard software. You should be able to start
Apache, PostgreSQL, Mozilla etc in the normal way, but because
everything is running off CD the performance will be very sluggish and
you may run out of memory for the 'tmpfs' file system. You may wish
to go ahead and install Xen/Xenlinux on your hard drive, either
dropping Xen and the Xenlinux kernel down onto a pre-existing Linux
distribution, or using the file systems from the CD (which are based
on RH9). See the installation instructions later in this document.
If your video card requires 'agpgart' then it unfortunately won't yet
work with Xen, and you'll only be able to configure a VGA X
server. We're working on a fix for this for the next release.
If you want to browse the Xen / Xenlinux source, it's all located
under /usr/local/src/xeno-1.3, complete with BitKeeper
repository. We've also included source code and configuration
information for the various benchmarks we used in the SOSP paper.
Starting other domains
======================
The first thing you need to do is to start the "xend" control daemon
with "xend start". You may wish to add an appropriate link to xend in
you /etc/rcX.d directory e.g. "ln -sf ../init.d/xend S97xend"
If you're not intending to configure the new domain with an IP address
on your LAN, then you'll probably want to use NAT. The
'xen_nat_enable' installs a few useful iptables rules into domain0 to
enable NAT. [NB: We plan to support RSIP in future]
Xen has a management interface that can be manipulated from domain0 to
create new domains, control their CPU, network and memory resource
allocations, allocate IP addresses, grant access to disk partitions,
and suspend/resume domains to files, etc. The management interface is
implemented as a set of library functions (implemented in C) for which
there are Python language bindings.
We have developed a simple set of example python tools for
manipulating the interface, with the intention that more sophisticated
high-level management tools will be developed in due course. Within
the source repository the tools live in tools/examples/ but are
installed in /usr/local/bin/ on the CD.
Starting a new domain is achieved using xc_dom_create.py which
allocates resources to a new domain, populates it with a kernel image
(and optionally a ramdisk) and then starts it.
It parses a configuration file written in the Python language, the
default location of which is "/etc/xc/defaults", but this may be
overridden with the "-f" option. For the Demo CD, the defaults file
will cause domains to be created with ram-based root file systems, and
mount their /usr partition from the CD, just like domain0. (If you are
writing your own config file, the "example" script may be a better
starting point)
Variables can be initialised and passed into configuration files. Some
of these may be compulsory, others optional.
The 'defaults' file on the CD requires the 'ip' variable to be set to
tell Xen what IP address(es) should be routed to this domain. Xen
will route packets to the domain if they bear one of these addresses
as a destination address, and will also ensure that packets sent from
the domain contain one of the addresses as a source address (to
prevent spoofing). If multiple IP addresses are to be assigned to a
domain they can be listed in a comma separated list (with no
whitespace).
The 'mem' variable can be used to change the default memory allocation
of 64MB. For example to start a domain with two IP addresses and
72MB:
xc_dom_create.py -Dip=128.23.45.34,169.254.1.1 -Dmem=72
[multiple variables may also be set with a single '-D' flag by
separating them with ':'. Also, it's possible to use DNS hostnames
rather than IP addresses.]
When invoked with the '-n' option xc_dom_create.py will do a dry run
and just print out what resources and configuration the domain will
have e.g.:
[root@xendemo]# xc_dom_create.py -D ip=commando-1.xeno,169.254.2.3 -Dmem=100
Parsing config file 'defaults'
VM image : "/boot/xenlinux.gz"
VM ramdisk : "/boot/initrd.gz"
VM memory (MB) : "100"
VM IP address(es) : "128.232.38.51:169.254.2.3"
VM block device(s) : "phy:cdrom,hdd,r"
VM cmdline : "ip=128.232.38.51:169.254.1.0:128.232.32.1:255.255.240.0::eth0:off root=/dev/ram0 rw init=/linuxrc 4 LOCALIP=169.254.2.3"
xc_dom_create.py will print the local TCP port to which you should
connect to perform console I/O. A suitable console client is provided
by the Python module xenctl.console_client: running this module from
the command line with <host> and <port> parameters will start a
terminal session. This module is also installed as /usr/bin/xencons,
from a copy in tools/misc/xencons. An alternative to manually running
a terminal client is to specify '-c' to xc_dom_create.py, or add
'auto_console=True' to the defaults file. This will cause
xc_dom_create.py to automatically become the console terminal after
starting the domain.
The 169.254.x.x network is special in that it is the 'link local'
subnet, and is isolated from the external network and hence can only
be used for communication between virtual machines. By convention, we
usually give each domain a link local address. The startup scripts on
the CD have been modified to accept a LINKLOCAL= parameter on the
kernel command line and initialise an IP alias accordingly (see
/etc/sysinit/network-scripts/ifcfg-eth0).
Linux only allows one IP address to be specified on the kernel command
line, so if you specify multiple IP addresses you'll need to configure
the new Linux VM with the other addresses manually (using ifconfig)
having logged in.
If you inspect the 'defaults' config script you'll see that the new
domain was started with a '4' on the kernel command line to tell
'init' to go to runlevel 4 rather than the default of 3 used by
domain0. This is done simply to suppress a bunch of harmless error
messages that would otherwise occur when the new (unprivileged) domain
tried to access physical hardware resources to try setting the
hwclock, system font, run gpm etc.
After it's booted, you should be able to ssh into your new domain from
domain0 using the link local 19.254.x.x address you assigned. If you
assigned a further IP address you should be able to ssh in using that
address too. If you ran the xen_enable_nat script, a bunch of port
redirects have been installed to enable you to ssh in to other domains
remotely even if you didn't assign an externally routeable address.
To access the new virtual machine remotely, use:
ssh -p2201 root@IP.address.Of.Domain0 # use 2202 for domain 2 etc.
You can manipulate running domains using the xc_dom_control.py tool.
Invoking it without arguments prints some usage information.
To see what domains are running, run 'xc_dom_control.py list'. Using the
tool you can change scheduling parameters, pause a domain, send it a
shutdown request, or blow it away with the 'destroy' command. You can
even suspend it to disk (but you probably won't have enough memory to
do the latter if you're running off the demo CD).
To find usage information for xc_dom_control.py, run the script with
no arguments.
Troubleshooting Problems
========================
If you have problems booting Xen, there are a number of boot parameters
that may be able to help diagnose problems:
ignorebiostables Disable parsing of BIOS-supplied tables. This may
help with some chipsets that aren't fully supported
by Xen. If you specify this option then ACPI tables are
also ignored, and SMP support is disabled.
noreboot Don't reboot the machine automatically on errors.
This is useful to catch debug output if you aren't
catching console messages via the serial line.
nosmp Disable SMP support.
This option is implied by 'ignorebiostables'.
noacpi Disable ACPI tables, which confuse Xen on some chipsets.
This option is implied by 'ignorebiostables'.
watchdog Enable NMI watchdog which can report certain failures.
noht Disable Hyperthreading.
badpage=<page number>[,<page number>]*
Specify a list of pages not to be allocated for use
because they contain bad bytes. For example, if your
memory tester says that byte 0x12345678 is bad, you would
place 'badpage=0x12345' on Xen's command line (i.e., the
last three digits of the byte address are not included!).
com1=<baud>,DPS[,<io_base>,<irq>]
com2=<baud>,DPS[,<io_base>,<irq>]
Xen supports up to two 16550-compatible serial ports.
For example: 'com1=9600,8n1,0x408,5' maps COM1 to a
9600-baud port, 8 data bits, no parity, 1 stop bit,
I/O port base 0x408, IRQ 5.
If the I/O base and IRQ are standard (com1:0x3f8,4;
com2:0x2f8,3) then they need not be specified.
console=<specifier list>
Specify the destination for Xen console I/O.
This is a comma-separated list of, for example:
vga: use VGA console and allow keyboard input
com1: use serial port com1
com2H: use serial port com2. Transmitted chars will
have the MSB set. Received chars must have
MSB set.
com2L: use serial port com2. Transmitted chars will
have the MSB cleared. Received chars must
have MSB cleared.
The latter two examples allow a single port to be
shared by two subsystems (eg. console and
debugger). Sharing is controlled by MSB of each
transmitted/received character.
[NB. Default for this option is 'com1,vga']
conswitch=<switch-char><auto-switch-char>
Specify how to switch serial-console input between
Xen and DOM0. The required sequence is CTRL-<switch_char>
pressed three times. Specifying '`' disables switching.
The <auto-switch-char> specifies whether Xen should
auto-switch input to DOM0 when it boots -- if it is 'x'
then auto-switching is disabled. Any other value, or
omitting the character, enables auto-switching.
[NB. Default for this option is 'a']
nmi=<nmi-error-behaviour>
Specify what to do with an NMI parity or I/O error.
'nmi=fatal': Xen prints a diagnostic and then hangs.
'nmi=dom0': Inform DOM0 of the NMI.
'nmi=ignore': Ignore the NMI.
[NB. Default is 'dom0' ('fatal' for debug builds).]
dom0_mem=xxx Set the initial amount of memory for domain0.
pdb=xxx Enable the pervasive debugger. See docs/pdb.txt
xxx defines how the gdb stub will communicate:
com1 use com1
com1H use com1 (with high bit set)
com2 use on com2
com2H use com2 (with high bit set)
It's probably a good idea to join the Xen developer's mailing list on
Sourceforge: http://lists.sourceforge.net/lists/listinfo/xen-devel
About The Xen Demo CD
=====================
The purpose of the Demo CD is to distribute a snapshot of Xen's
source, and simultaneously provide a convenient means for enabling
people to get experience playing with Xen without needing to install
it on their hard drive. If you decide to install Xen/Xenlinux you can
do so simply by following the installation instructions below -- which
essentially involves copying the contents of the CD on to a suitably
formated disk partition, and then installing or updating the Grub
bootloader.
This is a bootable CD that loads Xen, and then a Linux 2.4.22 OS image
ported to run on Xen. The CD contains a copy of a file system based on
the RedHat 9 distribution that is able to run directly off the CD
("live ISO"), using a "tmpfs" RAM-based file system for root (/etc
/var etc). Changes you make to the tmpfs will obviously not be
persistent across reboots!
Because of the use of a RAM-based file system for root, you'll need
plenty of memory to run this CD -- something like 96MB per VM. This is
not a restriction of Xen : once you've installed Xen, Xenlinux and
the file system images on your hard drive you'll find you can boot VMs
in just a few MBs.
The CD contains a snapshot of the Xen and Xenlinux code base that we
believe to be pretty stable, but lacks some of the features that are
currently still work in progress e.g. OS suspend/resume to disk, and
various memory management enhancements to provide fast inter-OS
communication and sharing of memory pages between OSs. We'll release
newer snapshots as required, making use of a BitKeeper repository
hosted on http://xen.bkbits.net (follow instructions from the project
home page). We're obviously grateful to receive any bug fixes or
other code you can contribute. We suggest you join the
xen-devel@lists.sourceforge.net mailing list.
Installing from the CD
======================
If you're installing Xen/Xenlinux onto an existing linux file system
distribution, just copy the Xen VMM (/boot/image.gz) and Xenlinux
kernels (/boot/xenlinux.gz), then modify the Grub config
(/boot/grub/menu.lst or /boot/grub/grub.conf) on the target system.
It should work on pretty much any distribution.
Xen is a "multiboot" standard boot image. Despite being a 'standard',
few boot loaders actually support it. The only two we know of are
Grub, and our modified version of linux kexec (for booting off a
XenoBoot CD -- PlanetLab have adopted the same boot CD approach).
If you need to install grub on your system, you can do so either by
building the Grub source tree
/usr/local/src/grub-0.93-iso9660-splashimage or by copying over all
the files in /boot/grub and then running /sbin/grub and following the
usual grub documentation. You'll then need to edit the Grub
config file.
A typical Grub menu option might look like:
title Xen / Xenlinux 2.4.26
kernel /boot/xen.gz dom0_mem=131072 com1=115200 noht watchdog
module /boot/vmlinuz-2.4.26-xen0 root=/dev/sda4 ro
The first line specifies which Xen image to use, and what command line
arguments to pass to Xen. In this case we set the maximum amount of
memory to allocate to domain0, and enable serial I/O on COM1 at 115200
baud. We could also disable smp support (nosmp) or disable
hyper-threading support (noht).
The second line specifies which xenlinux image to use, and the
standard linux command line arguments to pass to the kernel. In this
case, we're configuring the root partition and stating that it should
initially be mounted read-only (normal practice).
If we were booting with an initial ram disk (initrd), then this would
require a second "module" line.
Installing the Xen tools and source
===================================
The tools and source live in the /usr/local/src/xen-2.0 directory on
the CD (and may also be downloaded from the project downloads
page). You'll need to copy them to some mutable storage before using
them.
If you have the BitKeeper BK tools installed you can check the
repository is up to date by cd'ing into the xeno-2.0.bk directory and
typing 'bk pull' (assuming you have an Internet connection).
You can rebuild Xen, the tools and XenLinux by typing 'make
world'. You can install them to the standard directories with 'make
install', or into the ./install subtree with 'make dist'.
Modifying xc_mycreatelinuxdom1.py
=================================
xc_mycreatelinuxdom1.py.py can be used to set the new kernel's command line,
and hence determine what it uses as a root file system, etc. Although
the default is to boot in the same manner that domain0 did (using the
RAM-based file system for root and the CD for /usr) it's possible to
configure any of the following possibilities, for example:
* initrd=/boot/initrd init=/linuxrc
boot using an initial ram disk, executing /linuxrc (as per this CD)
* root=/dev/hda3 ro
boot using a standard hard disk partition as root
!!! remember to grant access in createlinuxdom.py.
* root=/dev/xvda1 ro
boot using a pre-configured 'virtual block device' that will be
attached to a virtual disk that previously has had a file system
installed on it.
* root=/dev/nfs nfsroot=/path/on/server ip=<blah_including server_IP>
Boot using an NFS mounted root file system. This could be from a
remote NFS server, or from an NFS server running in another
domain. The latter is rather a useful option.
A typical setup might be to allocate a standard disk partition for
each domain and populate it with files. To save space, having a shared
read-only usr partition might make sense.
Block devices should only be shared between domains in a read-only
fashion otherwise the linux kernels will obviously get very confused
as the file system structure may change underneath them (having the
same partition mounted rw twice is a sure fire way to cause
irreparable damage)! If you want read-write sharing, export the
directory to other domains via NFS from domain0.
Installing the file systems from the CD
=======================================
If you haven't got an existing Linux installation onto which you can
just drop down the Xen and Xenlinux images, then the file systems on
the CD provide a quick way of doing an install. However, you're
probably better off in the long run doing a proper Redhat, Fedora,
Debian etc install rather than just doing the hack described below:
Choose one or two partitions, depending on whether you want a separate
/usr or not. Make file systems on it/them e.g.:
mkfs -t ext3 /dev/hda3
[or mkfs -t ext2 /dev/hda3 && tune2fs -j /dev/hda3 if using an old
version of mkfs]
Next, mount the file system(s) e.g.:
mkdir /mnt/root && mount /dev/hda3 /mnt/root
[mkdir /mnt/usr && mount /dev/hda4 /mnt/usr]
To install the root file system, simply untar /usr/XenDemoCD/root.tar.gz:
cd /mnt/root && tar -zxpf /usr/XenDemoCD/root.tar.gz
You'll need to edit /mnt/root/etc/fstab to reflect your file system
configuration. Changing the password file (etc/shadow) is probably a
good idea too.
To install the usr file system, copy the file system from CD on /usr,
though leaving out the "XenDemoCD" and "boot" directories:
cd /usr && cp -a X11R6 etc java libexec root src bin dict kerberos local sbin tmp doc include lib man share /mnt/usr
If you intend to boot off these file systems (i.e. use them for
domain 0), then you probably want to copy the /usr/boot directory on
the cd over the top of the current symlink to /boot on your root
filesystem (after deleting the current symlink) i.e.:
cd /mnt/root ; rm boot ; cp -a /usr/boot .
The XenDemoCD directory is only useful if you want to build your own
version of the XenDemoCD (see below).
Debugging
=========
Xen has a set of debugging features that can be useful to try and
figure out what's going on. Hit 'h' on the serial line (if you
specified a baud rate on the Xen command line) or ScrollLock-h on the
keyboard to get a list of supported commands.
If you have a crash you'll likely get a crash dump containing an EIP
(PC) which, along with an 'objdump -d image', can be useful in
figuring out what's happened. Debug a Xenlinux image just as you
would any other Linux kernel.
We supply a handy debug terminal program which you can find in
/usr/local/src/xen-1.0/xeno-1.0.bk/tools/misc/miniterm/
This should be built and executed on another machine that is connected
via a null modem cable. Documentation is included.
Alternatively, if the Xen machine is connected to a serial-port server
then we supply a dumb TCP terminal client:
'tools/xenctl/lib/console_client.py <server host> <server port>'
Installing Xen / Xenlinux on a RedHat distribution
===================================================
When using Xen / Xenlinux on a standard Linux distribution there are
a couple of things to watch out for:
The first Linux VM that is started when Xen boots start (Domain 0) is
given direct access to the graphics card, so it may use it as a
console. Other domains don't have ttyN consoles, so attempts to run a
'mingetty' against them will fail, generating periodic warning
messages from 'init' about services respawning too fast. They should
work for domain0 just fine.
IMPORTANT: To prevent warning messages when running RH9 you'll need to
remove ttyN from /etc/inittab for domains>0. Due to a bug in the RH9
/etc/rc.sysinit script #'ing the lines out of /etc/inittab won't work
as it ignores the '#' and tries to access them anyway.
Every Xenlinux instance owns a bidirectional 'virtual console'.
The device node to which this console is attached can be configured
by specifying 'xencons=' on the OS command line:
'xencons=off' --> disable virtual console
'xencons=tty' --> attach console to /dev/tty1 (tty0 at boot-time)
'xencons=ttyS' --> attach console to /dev/ttyS0
The default is to attach to /dev/tty1, and also to create dummy
devices for /dev/tty2-63 to avoid warnings from many standard distro
startup scripts. The exception is domain 0, which by default attaches
to /dev/ttyS0.
Note that, because domains>0 don't have any privileged access at all,
certain commands in the default boot sequence will fail e.g. attempts
to update the hwclock, change the console font, update the keytable
map, start apmd (power management), or gpm (mouse cursor). Either
ignore the errors, or remove them from the startup scripts. Deleting
the following links are a good start: S24pcmcia S09isdn S17keytable
S26apmd S85gpm
If you want to use a single root file system that works cleanly for
domain0 and domains>0, one trick is to use different 'init' run
levels. For example, on the Xen Demo CD we use run level 3 for domain
0, and run level 4 for domains>0. This enables different startup
scripts to be run in depending on the run level number passed on the
kernel command line.
Xenlinux kernels can be built to use runtime loadable modules just
like normal linux kernels. Modules should be installed under
/lib/modules in the normal way.
If there's some kernel feature that hasn't been built into our default
kernel, there's a pretty good change that if its a non-hardware
related option you'll just be able to enable it and rebuild. If its
not on the xconfig menu, hack the arch/xen/config.in to put the menu
back in.
If you're going to use the link local 169.254.1.x addresses to
communicate between VMs, there are a couple of other issues to watch
out for. RH9 appears to have a bug where by default it configures the
loopback interface with a 169.254 address, which stops it working
properly on eth0 for communicating with other domains.
This utterly daft RH9 behaviour can be stopped by appending
"NOZEROCONF=yes" to /etc/sysconfig/networking-scripts/ifcfg-lo
If you're going to use NFS root files systems mounted either from an
external server or from domain0 there are a couple of other gotchas.
The default /etc/sysconfig/iptables rules block NFS, so part way
through the boot sequence things will suddenly go dead.
If you're planning on having a separate NFS /usr partition, the RH9
boot scripts don't make life easy, as they attempt to mount NFS file
systems way to late in the boot process. The easiest way I found to do
this was to have a '/linuxrc' script run ahead of /sbin/init that
mounts /usr:
#!/bin/bash
/sbin/ipconfig lo 127.0.0.1
/sbin/portmap
/bin/mount /usr
exec /sbin/init "$@" <>/dev/console 2>&1
The one slight complication with the above is that /sbib/portmap is
dynamically linked against /usr/lib/libwrap.so.0 Since this is in
/usr, it won't work. I solved this by copying the file (and link)
below the /usr mount point, and just let the file be 'covered' when
the mount happens.
In some installations, where a shared read-only /usr is being used, it
may be desirable to move other large directories over into the
read-only /usr. For example, on the XenDemoCD we replace /bin /lib and
/sbin with links into /usr/root/bin /usr/root/lib and /usr/root/sbin
respectively. This creates other problems for running the /linuxrc
script, requiring bash, portmap, mount, ifconfig, and a handful of
other shared libraries to be copied below the mount point. I guess I
should have written a little statically linked C program...
Description of how the XenDemoCD boots
======================================
1. Grub is used to load Xen, a Xenlinux kernel, and an initrd (initial
ram disk). [The source of the version of Grub used is in /usr/local/src]
2. the init=/linuxrc command line causes linux to execute /linuxrc in
the initrd.
3. the /linuxrc file attempts to mount the CD by trying the likely
locations : /dev/hd[abcd].
4. it then creates a 'tmpfs' file system and untars the
'XenDemoCD/root.tar.gz' file into the tmpfs. This contains hopefully
all the files that need to be mutable (this would be so much easier
if Linux supported 'stacked' or union file systems...)
5. Next, /linuxrc uses the pivot_root call to change the root file
system to the tmpfs, with the CD mounted as /usr.
6. It then invokes /sbin/init in the tmpfs and the boot proceeds
normally.
Building your own version of the XenDemoCD
==========================================
The 'live ISO' version of RedHat is based heavily on Peter Anvin's
SuperRescue CD version 2.1.2 and J. McDaniel's Plan-B:
http://www.kernel.org/pub/dist/superrescue/v2/
http://projectplanb.org/
Since Xen uses a "multiboot" image format, it was necessary to change
the bootloader from isolinux to Grub0.93 with Leonid Lisovskiy's
<lly@pisem.net> grub.0.93-iso9660.patch
The Xen Demo CD contains all of the build scripts that were used to
create it, so it is possible to 'unpack' the current iso, modifiy it,
then build a new iso. The procedure for doing so is as follows:
First, mount either the CD, or the iso image of the CD:
mount /dev/cdrom /mnt/cdrom
or:
mount -o loop xendemo-1.0.iso /mnt/cdrom
cd to the directory you want to 'unpack' the iso into then run the
unpack script:
cd /local/xendemocd
/mnt/cdrom/XenDemoCD/unpack-iso.sh
The result is a 'build' directory containing the file system tree
under the 'root' directory. e.g. /local/xendemocd/build/root
To add or remove rpms, its possible to use 'rpm' with the --root
option to set the path. For more complex changes, it easiest to boot a
machine using using the tree via NFS root. Before doing this, you'll
need to edit fstab to comment out the seperate mount of /usr.
One thing to watch out for: as part of the CD build process, the
contents of the 'rootpatch' tree gets copied over the existing 'root'
tree replacing various files. The intention of the rootpatch tree is
to contain the files that have been modified from the original RH
distribution (e.g. various /etc files). This was done to make it
easier to upgrade to newer RH versions in the future. The downside of
this is that if you edit an existing file in the root tree you should
check that you don't also need to propagate the change to the
rootpatch tree to avoid it being overwritten.
Once you've made the changes and want to build a new iso, here's the
procedure:
cd /local/xendemocd/build
echo '<put_your_name_here>' > Builder
./make.sh put_your_version_id_here >../buildlog 2>&1
This process can take 30 mins even on a fast machine, but you should
eventually end up with an iso image in the build directory.
Notes:
root - the root of the file system heirarchy as presented to the
running system
rootpatch - contains files that have been modified from the standard
RH, and copied over the root tree as part of the build
procedure.
irtree - the file system tree that will go into the initrd (initial
ram disk)
work - a working directory used in the build process
usr - this should really be in 'work' as its created as part of the
build process. It contains the 'immutable' files that will
be served from the CD rather than the tmpfs containing the
contents of root.tar.gz. Some files that are normally in /etc
or /var that are large and actually unlikely to need changing
have been moved into /usr/root and replaced with links.
Ian Pratt
9 Sep 2003
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