Xen Performance Guide
The best way to install the guest OS (DomU) is also not an obvious one. Most people have a RAID subsystem as storage for all the DomU virtual disks - but having this misaligned or not optimised can lead to significant performance degradation. This mostly comes about when having an LVM scheme where you use an LV to store the DomUs filesystem - but you use the ISO installer - so you end up offsetting everything by the partition table + boot data.
This means in most cases, when you write a block to the DomU\'s filesystem, it translates to two block writes in the physical storage array - you\'re essentially writing twice as much data as you need.
So, my way to solve this was to use the LV directly - with no partition table. We also start the installation via the Dom0 shell - that way the tools have full visibility as to the layout of the underlying block storage. This guide focuses on EL6 - specifically Scientific Linux 6. You can probably substitute easily enough for CentOS or RHEL.
1) Create a new LV to start with:
lvcreate -L 50G -n template.vm vg_domains
2) Create a filesystem on this LV:
mkfs.ext4 -j /dev/vg_domains/template.vm
3) Mount the newly created filesystem to /mnt/template:
mkdir -p /mnt/template mount /dev/vg_domains/template.vm /mnt/template
4) Now we want to create the bare directory template to allow installation.
mkdir -p /mnt/template/tmp /mnt/template/var/lib/rpm /mnt/template/var/log mkdir -p /mnt/template/dev /mnt/template/proc /mnt/template/sys
5) Create the new RPM database:
rpm --root=/mnt/template --initdb
6) Install the release file for the distro. In my case, I use the following:
wget http://ftp.scientificlinux.org/linux/scientific/6x/x86_64/os/Packages/sl-release-6.4-1.x86_64.rpm rpm -ivh --nodeps --root /mnt/template sl-release-6.4-1.x86_64.rpm
7) Install the base set of packages:
yum --installroot=/mnt/template -y groupinstall base
8) Now we can mount a few things and then chroot into the new filesystem to do what we need.
mount -o bind /dev /mnt/template/dev mount -o bind /proc /mnt/template/proc chroot /mnt/template
9) Install a kernel (this isn\'t included normally in the base group)
yum -y install kernel
10) Create a file: /boot/grub/grub.conf then populate it with the correct kernel versions (look in /boot and substitute where needed):
boot=/dev/sda default=0 timeout=1 splashimage=(hd0)/grub/splash.xpm.gz hiddenmenu title Scientific Linux (2.6.32-358.23.2.el6.x86_64) root (hd0) kernel /boot/vmlinuz-2.6.32-358.23.2.el6.x86_64 ro root=/dev/xvda rd_NO_LUKS rd_NO_DM LANG=en_AU.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=us crashkernel=auto console=hvc0 initrd /boot/initramfs-2.6.32-358.23.2.el6.x86_64.img
11) The last file we need to create is /etc/fstab. It will be something like this:
/dev/xvda / ext4 defaults 1 1 tmpfs /dev/shm tmpfs defaults 0 0 devpts /dev/pts devpts gid=5,mode=620 0 0 sysfs /sys sysfs defaults 0 0 proc /proc proc defaults 0 0
12) Our last real step to prepare the template now is to set a root password:
13) To configure the DomU now, we'll need to unmount everything:
exit umount /mnt/template/dev umount /mnt/template/proc umount /mnt/template
14) Create the Xen DomU config file as /etc/xen/template.vm:
memory = 512 name = "template.vm" vcpus = 1 disk = [ 'phy:/dev/vg_domains/template.vm,xvda,w' ] vif = [ "bridge=br0,script=vif-bridge" ] kernel="/usr/lib/xen/boot/pv-grub-x86_64.gz" extra="(hd0)/boot/grub/grub.conf"
15) Start the DomU and cross your fingers!
xl create /etc/xen/template.vm -c
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