Xen and the Art of System Administration
Johnny C. Lam
- I am not a kernel developer.
- My experience is in implementing systems and porting software.
- Covers Xen 2.0, which lacks some of the cooler Xen 3.0 features like
support for SMP domains.
- I only have a layman's knowledge of FreeBSD Jails.
How do we isolate processes and users?
- Processes and services shouldn't interfere with each other.
- Users shouldn't be able to stomp on each other.
- Users have different levels of access to services.
The Whole "UNIX" Thing
- Processes have their own address spaces.
- Processes can run as different users.
- Processes can run in chroot "jails".
- Filesystem access is managed via ACLs or permission masks.
We can run everything on a single machine by taking care with file and
directory permissions, using chroots, and having good bookkeeping skills.
- Bookkeeping overhead in documenting complex machine setups.
- Conflicting software installations require manual fix-ups.
- Delegation issues
- Allowing junior admins to manage some things but not others.
- Protecting against catastrophic screw-ups by junior admins.
- User/group access controls on resources are a pain in the butt.
- Windows 200x Active Directory anyone?
We can just run separate processes on separate machines.
- Easy to understand physical security.
- Can allocate processes to machines based on resource requirements, e.g.
more memory, faster disks, faster NICs, etc.
- Each machine only has the minimum number of local users needed.
Virtualization lets you do all this on a single machine.
- Dense -- save on hardware and power costs
- Simpler to maintain
- Easier to set up and tear down a virtual machine -- it's just
playing with bits.
- Don't need to learn or keep track of anything new or complex
admin-wise -- just treat a virtual machine like any other
- Need a beefy machine -- higher initial costs.
- Performance hit
Some virtualization technologies
These technologies provide varying degrees of virtualization:
Xen vs. Jails Deathmatch
Xen and jails are two completely different technologies, so comparisons
are unfair. However, both can be applied to solve a particular domain
of problems in system administration:
- Process isolation
- User isolation
- All on one machine
Xen provides "machine-level" virtualization, while jails provide "OS-level" virtualization and have different cost trade-offs.
Jails only exist on FreeBSD and DragonFly, and I use NetBSD, so I use Xen.
Originally developed by University of Cambridge Computer Lab, and currently developed by XenSource.
- GPL-licensed virtual machine monitor
- Xen hypervisor implements virtual x86 machines (with special devices)
- Securely execute multiple virtual machines with strict resource partitioning.
- Close-to-native performance
- domain 0
- Privileged domain
- Linux or NetBSD
- access to real hardware
- starts, stops & manages all guest domains
- domain U
- Unprivileged domains
- Linux, NetBSD, FreeBSD-5.x, Plan 9
- Only have access to block and network devices created by domain 0
- Dell PowerEdge 1750
- dual Xeon 3.2GHz processors
- 2GB RAM
- dual embedded GigE NICs
- PERC 4/Di embedded RAID with 3 150GB drives (RAID-5)
- domain 0
- NetBSD 3.0
- 64MB RAM
- Provide cgd-on-vnd devices for file-backed domains
- domain U
- NetBSD 3.0
- 128MB RAM
- varying amount of disk space
- file-backed domains
Domain 0 Setup
- Use IPfilter to block access to ports 8000, 8001, 8002
- control ports for xend, which allow managing guest domains.
- Create bridge(4) devices for each NIC
- Network interfaces for each guest domain are attached to a specific
- Mount USB key partition containing encryption keys for cgd(4) devices
- Start xend and all domains
Domain U Setup
- Each domain uses three partitions
- root.img (/)
- 512 MB, read-only & shared amongst all domU's
- Contains base installation of NetBSD 3.0
- Easy to update all domains to latest netbsd-3 branch
- pkg.img (/usr/pkg)
- 128+ MB, read-only
- contains pkgsrc-installed software
- Update all packages by swapping with new image
- Downgrade packages by swapping back with old image
- crypt.img (/crypt)
- 5+ GB, read-write
- encrypted partition
- contains server-specific data
- /crypt/etc and /crypt/var are null-mounted to /etc and /var
Example domain U configuration file
kernel = "/xen/netbsd-3/netbsd-XENU"
memory = 128
name = jabberwock
cpu = -1
nics = 1
vif = [ 'mac=ee:14:04:d0:ec:af, bridge=bridge0' ]
cmd = '/usr/pkg/etc/xen/block-file bind
out = os.popen(cmd)
vnd = out.readline().rstrip().rstrip('d') + 'a'
disk = [ 'cgd:' + vnd + ':/xen/cgd/jabberwock,wd2d,w',
root = "/dev/wd0d"
Example guest domains
- VPN router (configured across three NICs)
- Individual mail servers per DNS domain with a common pkg.img
- File server (a lot more disk)
- Web & Subversion server
- Database server (more RAM)
- pkgsrc development server
- package-installing server (to install packages into new pkg.img images
- It's quick and easy to test new software configurations on a scratch
- This Xen setup has similarities to setups for embedded devices.
- I can't wait till NetBSD can run on Xen 3.0.
- Breaks 2GB limit on memory
- Allows SMP guest domains for better processor utilitization
Links to more information