Quick provision script snap-guest for QEMU/KVM
Snap-guest is a simple script for creating copy-on-write QEMU/KVM guests.
Upstream site is at http://github.com/lzap/snap-guest
Features
- creates qcow2 image based on different one
- generates MAC address out of hostname
- modifies network settings (MAC, hostname) for Fedora/Red Hat distros
- disables fsck check during boot (qcow2 images can have lost inodes)
- creates and provisions guest using virt-install
- nice cli interface
Requirements
- bash :-)
- sed
- python-virtinst
- qemu-img
- libguestfs-mount
- vim (no, you don't really need it, but it's recommended :-)
Installation
- yum -y install bash sed python-virtinst qemu-img libguestfs-mount
- git clone git://github.com/lzap/snap-guest.git
- sudo ln -s $PWD/snap-guest/snap-guest /usr/local/bin/snap-guest
How it works
First of all you need to create base image using any method you want (e.g. virt-manager). It's recommended to use "base" string in the guest name (e.g. fedora-10-base or rhel4-base) to differentiate those files (snap-guest lists them using -l option), but it is not mandatory (option -a lists them all). The template image format can be qcow2 as well as different one (raw on LVM for example).
Feel free to configure the base image according to your needs. It's recommended to install a few packages like ntpd or acpid. Make sure network is also on when switching off NetworkManager. The following blog post contains more information about configuring base (or "template") guest:
http://lukas.zapletalovi.com/2011/08/configure-red-hat-or-fedora-as-guest.html
The only requirement is the hostname - it must be same as the base guest name. So if you name the VM fedora-10-base, hostname must be set the same.
The usage is very easy then:
usage: ./snap-guest options
Simple script for creating copy-on-write QEMU/KVM guests. For the base image
install Fedora or RHEL (compatible), install acpid and ntpd or similar, do not
make any swap partition (use -s option), make sure the hostname is the same
as the vm name and it has "base" in it. Example: rhel-6-base.
OPTIONS:
-h Show this message
-l List avaiable images (with "base" in the name)
-a List all images
-b [image] Base image name (template) - required
-t [image] Target image name (and hostname) - required
-n [network] Network settings (default: "network=default")
-m [MB] Memory (default: 800 MiB)
-c [CPUs] Number of CPUs (default: 1)
-p [path] Images path (default: /var/lib/libvirt/images/)
-d [domain] Domain suffix like "mycompany.com" (default: none)
-f Force creating new guest (no questions)
-w Add IP address to /etc/hosts (works only with NAT)
-s Swap size (in MB) that is appeded as /dev/sdb to fstab
-1 [command] Command to execute during first boot in /root dir
(logfile available in /root/firstboot.log)
EXAMPLE:
./snap-guest -l
./snap-guest -p /mnt/data/images -l
./snap-guest -b fedora-17-base -t test-vm -s 4098
./snap-guest -b fedora-17-base -t test-vm2 -n bridge=br0 -d example.com
./snap-guest -b rhel-6-base -t test-vm -m 2048 -c 4 -p /mnt/data/images
Snap-guest is a great tool for developing or testing. Provisioning a new guest from a template is very fast (about 5-10 seconds).
Warning
There is one important thing you need to know. Once you have some guests, you must not start a template image, because that would break the "child" guests.
You also must not change a template even when the "child" guests are not running. Again, if anything changes in a template, images based on the template will be corrupted. Sooner or later.
Trust me, it can seem to work since there is lot of files in a modern distribution (even a minimal installation). But the probability you corrupt some important files is very high. The template must not change when there are "child" guests - never ever.
The only safe way to change something in a template is to destroy all the "child" guests, change it and then re-provision them again. It's not big deal - it is fast, you know.
Network
The script modifies network settings in /etc/sysconfig directory (hostname and MAC address of the eth0). The MAC address is generated based on the hostname - the same hostname always gives the same address. Example:
hostname a => mac 52:54:00:60:b7:25
hostname b => mac 52:54:00:3b:5d:5c
hostname a => mac 52:54:00:60:b7:25 (the same)
This is great for testing - when you provision a box called let's say "test" and delete it, once it is provisioned again with the same name, DHCP will assign it the very same IP address. You can keep hostnames and IPs in the /etc/hosts file and if you won't be shut down your guests for longer periods, IPs never change.
Additionally, if you use snap-guest on the same host where KVM is running, there is a flag that adds entries to your /etc/hosts automatically. See help section for more details.
Recommended disk layout
Since snap-guest does not support LVM, you have to rely on the formatted partition. It is recommended to use separate dedicated partition for snap-guest. I am happy with ext4 using the extent option enabled and a bigger block size. Something like:
# pvdisplay
# lvdisplay
# lvcreate -L 140G -n lv_images vg_myhost
# mkfs.ext4 -b 4096 -O extent /dev/mapper/vg_myhost-lv_images
Credits and license
The script is distributed as public domain.
Original script was written by Red Hat folks (Jason Dobies, Shannon Hughes, Mike McCune and others), I have slightly modified it, I was using it and after few improvements I decided to share it with the world.
Special thanks to all who improve this set of scripts. See AUTHORS for full list.
vim: tw=79:fo+=w