Thursday Feb 24, 2011

VirtualBox and the Unbreakable Enterprise Kernel

This has tripped me up twice now, so time to write it down :-)

Oracle Linux 5 and now also Oracle Linux 6 come with a choice of kernels:

  • a 100% Red Hat compatible one; and
  • the Unbreakable Enterprise Kernel.

When installing VirtualBox, or the VirtualBox Guest Additions, we need to build and install kernel drivers which are dependent on the version of Linux you are using. So we need a few packages to be installed to allow us to do this.

Here's how using the standard kernel:

	yum update
	yum install gcc
	yum install kernel-devel

 or if using the Unbreakable kernel:

	yum update
	yum install gcc
	yum install kernel-uek-devel

 There, now I'll never forget how to do this again.


Wednesday Sep 08, 2010

VirtualBox and Linux kernel updates

In order to weave its magic, VirtualBox uses loadable kernel modules both on the Host, and in the Guest as part of the Guest Additions. On Linux, these kernel modules are related to the version of the kernel that you're running and if you update the kernel then the modules need to be rebuilt to match the new version. Technologies such as dkms handle this and so if you're running dkms you'll be in good shape. But if you aren't running dkms, then you may find that after an update and a reboot, VirtualBox or the Guest Additions don't run anymore. Here's how to fix things... 

If your Linux system is setup to build modules you can just execute one of:

  • Host: 
    /etc/init.d/vboxdrv setup
  • Guest:
    /etc/init.d/vboxadd setup

If your system is not setup to build kernel modules you will need to get the relevant packages to do this. For example, on Oracle Enterprise Linux you need to:

  1. Update your system to the latest version: open a terminal and as root, execute 
  2. yum update
  3. Install the GNU C compiler and the kernel development packages using
  4. yum install gcc 
    yum install kernel-devel
  5. Reboot your guest system in order to activate the updates before then running the commands above to rebuild the VirtualBox modules. 

 This is covered in more depth in the VirtualBox User Manual in the sections on Host Modules and Guest Additions.

Hope this is some use to someone


Wednesday Feb 10, 2010

Virtualization Product of the Year? ran a poll to find "Products of the Year" in various categories.

A pretty good result in the Virtualization section for VirtualBox - the People's Hypervisor ;-)

Thanks to all who voted. 


Tuesday Feb 09, 2010

Migrating from VMware to VirtualBox (Part 1): Oracle Enterprise Linux

There are a growing number of people asking the question: how do you move a VMware virtual machine to VirtualBox. So it is about time the Fat Bloke rolled up his sleeves and showed us how. (BTW you can click on screenshots below to magnify)

People typically want to do this because they have spent time installing a guest OS together with a software stack and they don't really want to go thru all this again. But moving a vm from one virtualization platform to another is analogous in the real world to unplugging a hard drive(s) from one computer and plugging it into a different manufacturer's computer. You may find that the guest operating system gets upset when it boots up and sees virtual hardware which is different than it was expecting. Different guest operating systems react differently to this situation (Linux is typically more accommodating than Windows in this respect).  In fact some guest operating systems get so upset, they may BSOD (Blue Screen of Death) on you.

Secondly, some of the software that you have installed above the OS, such as VMware Tools, may also be relying on specific virtual hardware.

So the Fat Bloke's First Rule of VM Migration is: Don't, if you can help it. If you can create a new vm from scratch on the new virtualization platform, you probably should. That way the guest OS installs the right drivers for your particular virtual hardware, and you are not left with orphaned software which needs a specific virtualization layer.

That said, there are still going to be people looking to avoid a complete reinstallation and willing to live dangerously, so let's discuss what is possible. Note that because different guests behave so differently we're going to focus on one guest OS in this blog: Oracle Enterprise Linux, and we'll move it from VMware Workstation 7 to VirtualBox 3.1. Here is our start state, the vm running in VMware Workstation 7 on Windows 7:

Starting Position

Step 1 - Preparing to Migrate 

It's a good idea to take a copy of the vm that we're trying to migrate just in case you make a mess of things. With VMware Workstation you can clone a vm to do this or copy the machine in some other way such as copying files.


To prepare for migration we're going to remove virtualization platform-specific components:

  1. Remove VMware tools
  2. Reset the Display and Input device  configuration
  3. Remove incompatible devices 

The Fat Bloke's Second Rule of VM Migration is that it is easier to unpick platform-specific software on the native virtualization platform. So let's start up the vm under VMware to prepare for migration

Removing VMware Tools 

This is easy enough:

and you should see something like this:

uninstall tools

Resetting the Display Device and Input Devices

When Oracle Linux was first installed the display was set as a VMware display adaptor and input devices as a VMware keyboard and mouse. By the time we're finished this won't be the case, so let's prepare for that by moving aside the the OEL conf file like this:

mv /etc/X11/xorg.conf /etc/X11/xorg.conf.vmware

This file will get recreated later when we run on the VirtualBox platform.

Now let's shutdown the guest. 


Remove Incompatible Devices

The VMware soundcard device is different from the one that VirtualBox exposes so let's remove this device from the vm configuration in the VMware Settings dialog:

Remove Soundcard

Step 2 - Exporting the Virtual Machine

A Virtual Machine consists of :

  • configuration information (in VMware a .vmx file)
  • virtual hard drive(s) which the guest is installed on. (in VMware, typically .vmdk files)

An emerging standard for encapsulating this information to allow vm's to be transported more easily is the OVF or Open Virtualization Format. So in theory you should be able to Export this vm from VMware Workstation and Import into VirtualBox. Sadly, the VMware conversion wizard (File...Import or Export...) doesn't support Oracle Enterprise Linux as a guest:

can't export

But there is a command line ovftool that can be downloaded from VMware's site.

So in a Windows command.exe window you can run:

cd C:\\Users\\fatbloke\\Documents\\Virtual Machines\\Clone of Oracle Enterprise Linux 
"\\Program Files\\VMware\\VMware OVF Tool"\\ovftool.exe "Clone of Oracle Enterprise Linux.vmx" OEL.ovf 

And eventually you end up with 3 files:

  1. OEL.ovf - configuration information
  2. OEL-disk1 - compressed disk format file
  3. -  digital signatures (SHA1) of the other files.

 We can now move these to the destination system for import by VirtualBox.

Step 3 - Importing into VirtualBox 

You can import an ovf file into VirtualBox from the graphical user interface or the command-line.

Import OVF

Importing this takes a while as the compressed disk is converted to a usable format, but your should end up with a new entry in your vm list in VirtualBox like this:

Imported vm

 Now the moment of truth, Start it up...and we should see Oracle Enterprise Linux boot up under VirtualBox

OEL under VirtualBox

 Step 4 - Install the VirtualBox Guest Additions

 Finally don't forget to install the VirtualBox Guest Additions which is the mirror image operation to removing the VMware Tools.

Install Guest Additions

Once you have mounted the Guest Additions iso image you can run install them from the mounted directory using the command:


 Like this:

Install GA

And after a restart of the guest you are all set to go.

Migration is complete! 

Epilogue (Advanced users only)

There is an alternative to step 2 and 3 above for people who know what they are doing.

After performing Step 1 you could simply take the VMware disk image (.vmdk)  and plug this into a manually configured VirtualBox vm. This effectively relies on the user to create an appropriately similar vm instead of relying on the ovf export and import (Steps 2 and 3 above) process. Note that the default disk controller used by VMware is an LSI SCSI controller, so when manually creating a VirtualBox vm you need to configure it appropriately. 

Add SCSI Controller

You still need Step 4 for optimal performance and ease of use. And don't forget the tip about Speeding up Linux Guests too.

Good Luck!

- FB 

Monday Jan 04, 2010

Speeding up your Linux Guests

With the clock ticking over to a new decade, now would seem to be a good time for a quick blog on timer interrupts in guests and how you can speed up your guests, while also lightening the load on your host, with the judicious use of a bit of guest configuration.

All operating systems make use of a system clock which ticks at a particular frequency. Common Linux distributions use kernels which drive the clock at 100Hz, 250Hz or 1000Hz. You can find out what your Linux kernel is configured for using this command:

grep CONFIG_HZ /boot/config-<kernel>

where kernel is the version of Linux you're running. The result of this command on my Oracle Enterprise Linux installation looks like this:

[root@localhost grub]# grep CONFIG_HZ /boot/config-2.6.18-164.el5
# CONFIG_HZ_100 is not set
# CONFIG_HZ_250 is not set

...which tells me that my kernel is configured to use a 1000Hz clock tick.

In a virtualized environment this means that there will be lots of context switches as the host schedules the guest to deal with clock ticks which don't do very much.  And this will become most visible by seeing a relatively high host cpu usage even when the guest is idle. (Note that the exact behaviour also depends on the host system. For example, the same OEL vm runs comfortably on my Mac host, but my Windows host gets very busy running it.)

If you see an idle Linux guest which is configured for a 1000Hz clock using up lots of host cpu cycles, you may want to reduce the clock frequency using the boot time parameter "divider=10". You can do this by adding the parameter manually as the grub boot loader starts, or by editing the grub configuration file as follows:

  1. Edit /boot/grub/grub.conf
  2. Duplicate the existing Title section, and rename it (this means you can choose at boot time which config to use)
  3. Add the "divider=10" parameter as follows:
 kernel /vmlinuz-2.6.18-164.el5 ro root=/dev/VolGroup01/LogVol00 rhgb quiet divider=10

Here is what my complete grub.conf looks like now:


This results in fewer context switches, a lighter host load (as measured by Window Task Manager) and faster guest execution. For example, the speed to boot my OEL vm (on a Windows 7 host) dropped from 115 seconds to 80 seconds which, according to my calculations, is a 30% increase in performance. Not bad for a simple bit of configuration ;-)



Fat Bloke


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