Running Systems

When one wants to achieve fast provisioning of virtual machines, some solutions might come into account. The one I prefer uses Linux LVM snapshot capabilities to duplicate one working machine into few.

This can happen, of course, only if the host running VMware-Server is Linux.

LVM snapshots have one vast disadvantage – performance. When a block on the source of the snapshot is being changed for the first time, the original block is being replicated to each and every snapshot COOW space. It means that a creation of a 1GB file on a volume having ten snapshots means a total copy of 10GB of data across your disks. You cannot ignore this performance impact.

LVM2 has support for read/write snapshots. I have come up with a nice way of utilizing this capability to my benefit. An R/W snapshot which is being changed does not replicate its changes to any other snapshot. All changes are considered local to this snapshot, and are being maintained only in its COOW space. So adding a 1GB file to a snapshot has zero impact on the rest of the snapshots or volumes.

The idea is quite simple, and it works like this:

1. Create adequate logical volume with a given size (I used 9GB for my own purposes). The name of the LV in my case will be /dev/VGVM3/centos-base

2. Mount this LV on a directory, and create a VM inside it. In my case, it’s in /vmware/centos-base

3. Install the VM as the baseline for all your future VMs. If you might not want Apache on some of them, don’t install it on the baseline.

4. Install vmware-tools on the baseline.

5. Disable the service “kudzu”

6. Update as required

7. In my case I always use DHCP. You can set it to obtain its IP once from a given location, or whatever you feel like.

8. Shut down the VM.

9. In the VM’s .vmx file add a line like this:

uuid.action = “create”

I have added below (expand to read) two scripts which will create the snapshot, mount it and register it, including new MAC and UUID.

Press below for the scripts I have used to create and destroy VMs

create-replica.sh:

#!/bin/sh
# This script will replicate vms from a given (predefined) source to a new system
# Written by Ez-Aton, http://www.tournament.org.il/run
# Arguments: name

# FUNCITONS BE HERE
test_can_do () {
# To be able to snapshot, we need a set of things to happen
if [ -d $DIR/$TARGET ] ; then
echo “Directory already exists. You don’t want to do it…”
exit 1
fi
if [ -f $VG/$TARGET ] ; then
echo “Target snapshot exists”
exit 1
fi
if [ `vmrun list | grep -c $DIR/$SRC/$SRC.vmx` -gt “0” ] ; then
echo “Source VM is still running. Shut it down before proceeding”
exit 1
fi
if [ `vmware-cmd -l | grep -c $DIR/$TARGET/$SRC.vmx` -ne “0” ] ; then
echo “VM already registered. Unregister first”
exit 1
fi
}

do_snapshot () {
# Take the snapshot
lvcreate -s -n $TARGET -L $SNAPSIZE $VG/$SRC
RET=$?
if [ “$RET” -ne “0” ]; then
echo “Failed to create snapshot”
exit 1
fi
}

mount_snapshot () {
# This function creates the required directories and mounts the snapshot there
mkdir $DIR/$TARGET
mount $VG/$TARGET $DIR/$TARGET
RET=$?
if [ “$RET” -ne “0” ]; then
echo “Failed to mount snapshot”
exit 1
fi
}

alter_snap_vmx () {
# This function will alter the name in the VMX and make it the $TARGET name
cat $DIR/$TARGET/$SRC.vmx | grep -v “displayName” > $DIR/$TARGET/$TARGET.vmx
echo “displayName = “$TARGET”” >> $DIR/$TARGET/$TARGET.vmx
cat $DIR/$TARGET/$TARGET.vmx > $DIR/$TARGET/$SRC.vmx
rm $DIR/$TARGET/$TARGET.vmx
}

register_vm () {
# This function will register the VM to VMWARE
vmware-cmd -s register $DIR/$TARGET/$SRC.vmx
}

# MAIN
if [ -z “$1” ]; then
echo “Arguments: The target name”
exit 1
fi

# Parameters:
SRC=centos-base         #The name of the source image, and the source dir
PREFIX=centos             #All targets will be created in the name centos-$NAME
DIR=/vmware               #My VMware VMs default dir
SNAPSIZE=6G              #My COOW space
VG=/dev/VGVM3           #The name of the VG
TARGET=”$PREFIX-$1″

test_can_do
do_snapshot
mount_snapshot
alter_snap_vmx
register_vm
exit 0

remove-replica.sh:

#!/bin/sh
# This script will remove a snapshot machine
# Written by Ez-Aton, http://www.tournament.org.il/run
# Arguments: machine name

#FUNCTIONS
does_it_exist () {
# Check if the described VM exists
if [ `vmware-cmd -l | grep -c $DIR/$TARGET/$SRC.vmx` -eq “0” ]; then
echo “No such VM”
exit 1
fi
if [ ! -e $VG/$TARGET ]; then
echo “There is no matching snapshot volume”
exit 1
fi
if [ `lvs $VG/$TARGET | awk ‘{print $5}’ | grep -c $SRC` -eq “0” ]; then
echo “This is not a snapshot, or a snapshot of the wrong LV”
exit 1
fi
}

ask_a_thousand_times () {
# This function verifies that the right thing is actually done
echo “You are about to remove a virtual machine and an LVM. Details:”
echo “Machine name: $TARGET”
echo “Logical Volume: $VG/$TARGET”
echo -n “Are you sure? (y/N): “
read RES
if [ “$RES” != “Y” ]&&[ “$RES” != “y” ]; then
echo “Decided not to do it”
exit 0
fi
echo “”
echo “You have asked to remove this machine”
echo -n “Again: Are you sure? (y/N): “
read RES
if [ “$RES” != “Y” ]&&[ “$RES” != “y” ]; then
echo “Decided not to do it”
exit 0
fi
echo “Removing VM and snapshot”
}

shut_down_vm () {
# Shut down the VM and unregister it
vmware-cmd $DIR/$TARGET/$SRC.vmx stop hard
vmware-cmd -s unregister $DIR/$TARGET/$SRC.vmx
}

remove_snapshot () {
# Umount and remove the snapshot
umount $DIR/$TARGET
RET=$?
if [ “$RET” -ne “0” ]; then
echo “Cannot umount $DIR/$TARGET”
exit 1
fi
lvremove -f $VG/$TARGET
RET=$?
if [ “$RET” -ne “0” ]; then
echo “Cannot remove snapshot LV”
exit 1
fi
}

remove_dir () {
# Removes the mount point
rmdir $DIR/$TARGET
}

#MAIN
if [ -z “$1” ]; then
echo “No machine name. Exiting”
exit 1
fi

#PARAMETERS:
DIR=/vmware                #VMware default VMs location
VG=/dev/VGVM3            #The name of the VG
PREFIX=centos              #Prefix to the name. All these VMs will be called centos-$NAME
TARGET=”$PREFIX-$1″
SRC=centos-base           #The name of the baseline image, LVM, etc. All are the same

does_it_exist
ask_a_thousand_times
shut_down_vm
remove_snapshot
remove_dir

exit 0

Pros:

1. Very fast provisioning. It takes almost five seconds, and that’s because my server is somewhat loaded.

2. Dependable: KISS at its marvel.

3. Conservative on space

4. Conservative on I/O load (unlike the traditional use of LVM snapshot, as explained in the beginning of this section).

Cons:

1. Cannot streamline the contents of snapshot into the main image (LVM team will implement it in the future, I think)

2. Cannot take a snapshot of a snapshot (same as above)

3. If the COOW space of any of the snapshots is full (viewable through the command ‘lvs‘) then on boot, the source LV might not become active (confirmed RH4 bug, and this is the system I have used)

4. My script does not edit/alter /etc/fstab (I have decided it to be rather risky, and it was not worth the effort at this time)

5. My script does not check if there is enough available space in the VG. Not required, as it will fail if creation of LV will fail

You are most welcome to contribute any further changes done to this script. Please maintain my URL in the script if you decide to use it.

Thanks!

For the latest (and currently whole) ks.cfg I use, check this link. I have extended the logic there, and got the following out of it. Showing only the %pre section:

%pre
# By Ez-Aton http://www.tournament.org.il/run
for i in `cat /proc/cmdline`; do
echo $i >> /tmp/vars.tmp
done
grep “=” /tmp/vars.tmp > /tmp/vars
# Parse command line. Using only vars with type var=value (doesn’t matter
# what the actual value is)
KS=/tmp/ks.cfg
update=””
name=””
pkg=””
. /tmp/vars
# Shall we update the system during the %post section?
if [ ! -z “$update” ]; then
echo “yum update -y” >> $KS
fi
# Shall we reboot the system after the installation?
if [ ! -z “$reboot” ]; then
echo “reboot” > $KS.tmp
cat $KS >> $KS.tmp
cat $KS.tmp > $KS
fi
# What is the machine’s hostname?
if [ ! -z “$name” ]; then
value=”dhcp –hostname $name”
cat $KS | sed s/dhcp/”$value”/ > $KS.tmp
cat $KS.tmp > $KS
fi
# Shall we add another package to the installation preset?
if [ ! -z “$pkg” ]; then
pkg_line=`grep -n ^%packages $KS | cut -f 1 -d :`
max_line=`wc -l $KS | awk ‘{print $1}’`
head -n $pkg_line $KS > $KS.tmp
for i in `echo $pkg | sed s/,/ /g`; do
echo $i >> $KS.tmp
done
let tail_line=$max_line-$pkg_line
tail -n $tail_line $KS >> $KS.tmp
cat $KS.tmp > $KS
fi
# Is it a virtual machine running on VMWare? If so, we’ll install vmware-tools
if [ ! -z “$vmware” ]; then
# We need vmhttp value for server. It can be the name and path
# of the web server
if [ -z “$vmhttp” ]; then
# my defaults
vmhttp=”centos4-01″
fi
# The name of the rpm is always vmware.rpm
echo “wget http://$vmhttp/vmware.rpm” >> $KS
echo “rpm -i vmware.rpm” >> $KS
fi