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@@ -8,203 +8,25 @@ __ __ ____ ___ ############################### University of Cambridge Computer Laboratory -28 October 2004 +29 October 2004 -http://www.cl.cam.ac.uk/netos/xen +http://www.cl.cam.ac.uk/netos/xen/ About the Xen Virtual Machine Monitor ===================================== -"Xen" is a Virtual Machine Monitor (VMM) originally developed by the +Xen is a Virtual Machine Monitor (VMM) originally developed by the Systems Research Group of the University of Cambridge Computer -Laboratory as part of the UK-EPSRC funded XenoServers project. +Laboratory, as part of the UK-EPSRC funded XenoServers project. -The XenoServers project aims to provide a "public infrastructure for -global distributed computing", and Xen plays a key part in that, -allowing us to efficiently partition a single machine to enable -multiple independent clients to run their operating systems and -applications in an environment providing protection, resource -isolation and accounting. The project web page contains further -information along with pointers to papers and technical reports: -http://www.cl.cam.ac.uk/xeno +The 2.0 release offers excellent performance, hardware support and +enterprise-grade features such as live migration. Linux 2.6, 2.4 and +NetBSD 2.0 are already available for Xen, with more operating system +ports on the way. -Xen has since grown into a project in its own right, enabling us to -investigate interesting research issues regarding the best techniques -for virtualizing resources such as the CPU, memory, disk and network. -The project has been bolstered by support from Intel Research -Cambridge, and HP Labs, who are now working closely with us. - -Xen enables multiple operating system images to execute concurrently -on the same hardware with very low performance overhead --- much lower -than commercial offerings for the same x86 platform. - -This is achieved by requiring OSs to be specifically ported to run on -Xen, rather than allowing unmodified OS images to be used. Crucially, -only the OS needs to be changed -- all of the user-level application -binaries, libraries etc can run unmodified. Hence the modified OS -kernel can typically just be dropped into any existing OS distribution -or installation. - -Xen currently runs on the x86 architecture, but could in principle be -ported to others. In fact, it would have been rather easier to write -Xen for pretty much any other architecture as x86 is particularly -tricky to handle. A good description of Xen's design, implementation -and performance is contained in our October 2003 SOSP paper, available -at http://www.cl.cam.ac.uk/netos/papers/2003-xensosp.pdf -[update: work to port Xen to x86_64 and IA64 is underway] - -To date five different operating systems have been ported to run on -Xen 2.0: Linux 2.4/2.6, NetBSD, FreeBSD and Plan 9. - -The Linux 2.4 and 2.6 ports works very well -- we regularly use them -to host complex applications such as PostgreSQL, Apache, BK servers -etc. It runs every user-space applications we've tried. We refer to -our version of Linux ported to run on Xen as "XenLinux", although -really it's just standard Linux ported to a new virtual CPU -architecture that we call xen-x86. - -NetBSD has been ported to Xen by Christian Limpach, and will hopefully -soon become part of the standard release. Work on a FreeBSD port has -been started by Kip Macy, and we hope to see this complete for the 2.0 -release. Ron Minnich has been working on Plan 9. - -A version of Windows XP was ported to an earlier version of Xen, but -can not be released due to licensing restrictions. We are -investigating alternative approaches to getting Windows XP/2003 -running on Xen, but have no firm timetable as yet. - -So, for the moment, you only get to run Linux 2.4/2.6 and NetBSD on -Xen, but we hope this will change before long. Even running multiple -copies of the same OS can be very useful, as it provides a means of -containing faults to one OS image, and also for providing performance -isolation between the various OS, enabling you to either restrict, or -reserve resources for, particular VM instances. - -It's also useful for development -- each version of Linux can have -different patches applied, enabling different kernels to be tried -out. For example, the "vservers" patch used by PlanetLab applies -easily to our ported version of Linux. - -We've successfully booted over 128 copies of Linux on the same machine -(a dual CPU hyperthreaded Xeon box) but we imagine that it would be -more normal to use some smaller number, perhaps 10-20. - -A common question is "how many virtual machines can I run on hardware -xyz?". The answer is very application dependent, but the rule of thumb -is that you should expect to be able to run the same workload under -multiple guest OSs that you could run under a single Linux instance, -with an additional overhead of a few MB per OS instance. - -One key feature in this new release of Xen is `live migration'. This -enables virtual machines instances to be dynamically moved between -physical Xen machines, with typical downtimes of just a few tens of -milliseconds. This is really useful for admins that want to take a -node down for maintenance, or to load balance a large number of -virtual machines across a cluster. - - -Hardware support -================ - -Xen is intended to be run on server-class machines, and the current -list of supported hardware very much reflects this, avoiding the need -for us to write drivers for "legacy" hardware. It is likely that some -desktop chipsets will fail to work properly with the default Xen -configuration: specifying 'noacpi' or 'ignorebiostables' when booting -Xen may help in these cases. - -Xen requires a "P6" or newer processor (e.g. Pentium Pro, Celeron, -Pentium II, Pentium III, Pentium IV, Xeon, AMD Athlon, AMD Duron). -Multiprocessor machines are supported, and we also have basic support -for HyperThreading (SMT), although this remains a topic for ongoing -research. We're also working on an AMD x86_64 port (though Xen should -run on Opterons in 32-bit mode just fine). - -Xen can currently use up to 4GB of memory. It's possible for x86 -machines to address more than that (64GB), but it requires using a -different page table format (3-level rather than 2-level). We -currently don't support the 3-level format, as we are concentrating -our efforts on an x86_64 port that will provide a better solution to -large memory configurations. - -In contrast to previous Xen versions, in Xen 2.0 device drivers run -within a privileged guest OS rather than within Xen itself. This means -that we should be compatible with almost the full set of device -hardware supported by Linux. The default XenLinux build contains -support for relatively modern server-class network and disk hardware, -but you can add suppport for other hardware by configuring your -XenLinux kernel in the normal way (e.g. "make xconfig"). - - -Building Xen and XenLinux -========================= - -The public master BK repository for the 2.0 release lives at: -bk://xen.bkbits.net/xen-2.0.bk - -To fetch a local copy, install the BitKeeper tools, then run: -'bk clone bk://xen.bkbits.net/xen-2.0.bk' - -You can do a complete build of Xen, the control tools, and the -XenLinux kernel images with "make world". This can take 10 minutes -even on a fast machine. If you're on an SMP machine you may wish to -give the '-j4' argument to make to get a parallel build. All of the -files that are built are placed under the ./install directory. You -can then install everything to the standard system directories -(e.g. /boot, /usr/bin, /usr/lib/python/ etc) by typing "make install". - -Take a look in install/boot/: - install/boot/xen.gz The Xen 'kernel' (formerly image.gz) - install/boot/vmlinuz-2.4.27-xen0 Domain 0 XenLinux kernel (xenolinux.gz) - install/boot/vmlinuz-2.4.27-xenU Unprivileged XenLinux kernel - -The difference between the two Linux kernels that are built is -due to the configuration file used for each. The "U" suffixed -unprivileged version doesn't contain any of the physical hardware -device drivers, so is 30% smaller and hence may be preferred for -your non-privileged domains. - -The install/boot directory will also contain the config files -used for building the XenLinux kernels, and also versions of Xen -and XenLinux kernels that contain debug symbols (xen-syms and -vmlinux-syms-2.4.27-xen0) which are essential for interpreting crash -dumps. - -Inspect the Makefile if you want to see what goes on during a -build. Building Xen and the tools is straightforward, but XenLinux is -more complicated. The makefile needs a 'pristine' linux kernel tree -which it will then add the Xen architecture files to. You can tell the -makefile the location of the appropriate linux compressed tar file by -setting the LINUX_SRC environment variable -(e.g. "LINUX_SRC=/tmp/linux-2.4.27.tar.gz make world") or by placing -the tar file somewhere in the search path of LINUX_SRC_PATH which -defaults to ".:..". If the makefile can't find a suitable kernel tar -file it attempts to download it from kernel.org, but this won't work -if you're behind a firewall. - -After untaring the pristine kernel tree, the makefile uses the -'mkbuildtree' script to add the Xen patches the kernel. "make world" -then build two different XenLinux images, one with a "-xen0" extension -which contains hardware device drivers and is intended to be used in -the first virtual machine ("domain 0"), and one with a "-xenU" -extension that just contains virtual-device drivers. The latter can be -used for all non hardware privileged domains, and is substantially -smaller than the other kernel with its selection of hardware drivers. - -If you don't want to use bitkeeper to download the source, you can -download prebuilt binaries and src tar balls from the project -downloads page: http://www.cl.cam.ac.uk/netos/xen/downloads/ - - -Using the domain control tools -============================== - -Before starting domains you'll need to start the node management -daemon: "xend start". -The primary tool for starting and controlling domains is "xm". -"xm help <cmd>" will tell you how to use it. - -Further documentation is in the docs/ directory. Postscript, PDF and -HTML versions of the user manual can be found in the ps/, pdf/ and -html/ subdirectories. +Xen is freely-distributable Open Source software, released under the +GNU GPL. +For full documentation, see the Xen User Manual in docs/pdf/user.pdf +(after running make -C docs) or the Documentation page on the Xen +website. |