First Impressions with Xen

I've been a VMware boy for years. I started with at least Workstation 3, probably Workstation 2. I think they have an excellent product. VMware Infrastructure is quite amazing and VMware server is a great free product.

I've been half heartedly trying to get Xen vm's up for the past couple of weeks. I haven't really read documentation -- I've mostly been googling around for instructions. Most of what I have done is based on the CentOS Wiki.

I finally got a VM going this morning. I ran into trouble in a few areas. First, I couldn't get xen to mount virtual disks. That problem turned out to be a corrupted install.

My second problem had to do with network interfaces. The VM would boot, but it couldn't find the network card to do the install over. Some of the documentation said to set vif0= in the config file. It turns out that in the version I'm using, it's vif=. I don't know if it changed recently, but changing it solved my problem.

So, this is what I did:

1. On my CentOS 5 machine, I made sure that xen-libs, kernel-xen, xen-devel and xen were all installed.
2. I downloaded the kernels from http://mirror.centos.org/centos/5/os/i386/images/xen/ and put them in /var/xen/kernels/i386/
3. I created a new lvm partition, /var/xen/, to hold images and kernels, etc.
4. I made sure xend was set to boot on start (chkconfig --level 2345 xend on)
5. I rebooted, making sure I was booting into the xen kernel
6. I created my disk image with dd if=/dev/zero of=/var/xen/test.img oflag=direct bs=1M seek=4096 count=1
   
7. I created a kickstart file like the one below and put it on a webserver:
   
install
url --url http://mirror.centos.org/centos/5/os/i386
lang en_US.UTF-8
network --device eth0 --bootproto dhcp
# Bogus password, change to something sensible!
rootpw bogus
firewall --enabled --port=
authconfig --enableshadow --enablemd5
selinux --enforcing −−port=22:tcp
timezone --utc Europe/Amsterdam
bootloader --location=mbr --driveorder=xvda --append="console=xvc0"
reboot

# Partitioning
clearpart --all --initlabel --drives=xvda
part /boot --fstype ext3 --size=100 --ondisk=xvda
part pv.2 --size=0 --grow --ondisk=xvda
volgroup VolGroup00 --pesize=32768 pv.2
logvol / --fstype ext3 --name=LogVol00 --vgname=VolGroup00 --size=1024 --grow
logvol swap --fstype swap --name=LogVol01 --vgname=VolGroup00 --size=256 --grow --maxsize=512

%packages
@core

8. The xen install config file looks like
   kernel = "/var/xen/kernels/i386/vmlinuz"
ramdisk = "/var/xen/kernels/i386/initrd.img"
extra = "text ks=http://the webserver you uploaded to/test.cfg"
name = "test"
memory = "512"
disk = [ 'tap:aio:/var/xen/test.img,xvda,w' ]
vif = [ '' ]
vcpus=2
on_reboot = 'destroy'
on_crash = 'destroy'

   Put it in /etc/xen/test. Be sure to replace the correct webserver name.
9. Start the install with xm start test. You can watch it with xm console test
10. When it's done, overwrite your config file with the following:
    
name = "test"
memory = "512"
disk = [ 'tap:aio:/var/xen/test.img,xvda,w' ]
vif = [ '' ]
vcpus=2
on_reboot = 'restart'
on_crash = 'restart'
bootloader="/usr/bin/pygrub"
11. Boot it back up with xm create test. When you want to turn it off, run xm destroy test. That doesn't actually destroy your file, it only turns off the vm.

BTW, I highly recommend *not* running /etc/xen/scripts/network-bridge stop on a remote system. Bad things will happen. Lesson learned.

Comparing to VMware Server, xen seems way better. VMware Server doesn't handle load well and tends to make everything slow on a moderately loaded server. That is why I wanted to give Xen a go instead. Xen feels a lot more like ESX (which makes sense being a similar sort of hypervisor).