KVM Virtual Machine Manager and Virtual Machines on external drives

Last year, I blogged about my first experiences with KVM and Virtual Machine Manager.

Then, I stopped using KVM because I’ve always found VirtualBox easier for my experiments. In particular, with VirtualBox, it is trivial to store virtual machines on an external drive (I mean, a fast external SD, of course): you specify to use a directory on the external drive, and all information about the virtual machine will be stored there. Then, you attach the drive to another computer with VirtualBox and open the virtual machine from the external drive. Easy!

Things are more complicated with KVM, QEMU, and Virtual Machine Manager. Even making QEMU access an external drive requires additional configuration steps.

In this blog post, I’ll summarize the steps to achieve that.

I’ll first show the manual export/import procedure for the machines’ metadata information. Then, I’ll show a different approach based on symlinks.

It was time to try KVM again because it’s faster than VirtualBox.

I’ll describe the installation steps for EndeavourOS and pure Arch Linux. I guess the steps for other distributions are similar.

Installation and configuration

Let’s install a few packages for KVM, QEMU, and the Virtual Machine Manager:

If you get this message, accept to remove “iptables”:

To use your user without entering the root password, we need to edit the file “/etc/libvirt/libvirtd.conf” and uncomment the following lines:

Or, append them at the end of the file:

Add your user account to the “libvirt” group.

Now comes the crucial part for letting QEMU handle machines on external drives: we need to add our user to “/etc/libvirt/qemu.conf”. This can be done by setting the appropriate entries in the file or by simply appending the entries at the end of the file:

If you want to start the virtualization service and the default virtual network automatically at boot, you run:

Since I’m not using virtual machines daily, I prefer to start them when needed, so I don’t run the above commands. Of course, I must remember to run these commands (note, for the network is “start” instead of “autostart”) before starting the “Virtual Machine Manager”:

Remember you can always use:

to see the service status and possible errors shown when running this command.

OK, time to reboot now.

Let’s create a virtual machine on an external drive

I created a directory “kvm/images” on my external USB SD to store the virtual machine images.

Let’s start the “Virtual Machine Manager” program. We should see “QEMU/KVM”:

Let’s create a new virtual machine with the leftmost toolbar button.

I specify a local ISO.

I don’t create a pool for ISOs and use “Browse Local” to select an ISO in my external drive.

In this example, I will install EndeavourOS on the virtual machine. I have to select the operating system manually (start typing, and you get completions):

Time to allocate resources for the virtual machine. I’m giving the VM half my RAM and half my cores:

Now here’s the essential part of disk selection. Remember, I want to use my external drive, so I select custom storage and press “Manage”:

In the following dialog, I use the “+” button in the bottom left corner to create a new pool:

I give the pool the name “images” and specify the directory I mentioned above on my external drive:

After pressing “Finish”, I select the created pool and add a “Volume” (with the other “+” button)

I give the disk image a proper name and enough size (recall that the image will NOT allocate all the size immediately, but only on-demand):

Select the volume and press “Choose Volume”:

On the final dialog, make sure the default network is selected and that you check “Customize configuration before install” (note that I also changed the name for the virtual machine):

Let’s press “Finish,” and get to the configuration dialog. I changed the Firmware from “BIOS” to “UEFI”, pressed “Apply,” and finally, we can start the installation with “Begin Installation”.

We should not get any error from QEMU because it cannot access the external drive, thanks to the configuration shown above in the qemu.conf file!

After the GRUB menu, we should see the installer log:

And then, the EndeavourOS installer dialog:

Since I’ve already blogged about EndeavourOS installation, I’ll skip the detailed steps. I’ll install the GNOME desktop environment and let the installer use the whole disk space with the BTRFS filesystem and SWAP with hibernate (later, I might want to check whether hibernate works in the VM).

In a few minutes, the installation finishes! We get to the GRUB menu of the installed system:

And to the installed GNOME desktop:

The disk image is correctly created in the external drive:

And the information about the virtual machine is in the:

Export the virtual machine

First, let’s shut down the machine.

Let’s export the virtual machine to use it from another computer. I understand that having the same software on the other host is crucial. Since I’m using EndeavourOS or Arch on my main computers, that is not a problem.

But isn’t the virtual machine already in an external drive? Why do I have to export it?

That’s the main difference with VirtualBox I mentioned at the beginning. The disk image is on an external drive, but the virtual machine information (configuration and metadata) is on a local XML file (see the listing of “/etc/libvirt/qemu” above; the XML file of the virtual machine is “eos-kvm-gnome.xml”, after the name I gave to the virtual machine when I created it).

Remember that the XML has an absolute path pointing to the disk image on the external drive:

So, again, in the other computers, the mount point of the external drive must be the same; otherwise, the absolute path must be manually adapted.

We could copy the XML file directly on the external drive (somewhere near the disk image to be easily found), e.g.:

Alternatively, if we don’t remember the location of the XML file, we can use the “dump” command.

For example, we can first list the current machines (in the example, I have only one):

And then, we dump its XML configuration:

We’re ready to import and use the VM on another computer

Import the virtual machine

I have already installed and configured KVM on another computer, following the same procedure at the beginning of the post.

Since I haven’t enabled the services at boot time, I run the following:

I connect the external drive and ensure it’s mounted (remember, on the same mount point as in the other computer).

Then, I create the virtual machine information locally by using the XML file on the drive I created above:

We can verify that the XML is now in the directory of QEMU:

Let’s start “Virtual Machine Manager,” and we can see the virtual machine:

We can start it, and it should work as on the other computer.

Cloning and Snapshots

Let’s create a clone of this virtual machine, e.g., with the context menu of the machine in the main user interface.

The destination path is based on the path of the current machine, the external drive, which is good.

Let’s wait for the clone to finish, and then we have two virtual machines:

If I want this clone to be usable on other computers, I repeat the export procedure for this new virtual machine:

I’ll leave this clone virtual machine as it is for now, and I’ll create a snapshot in the other virtual machine, the original one.

Snapshot information is stored somewhere else, NOT in the XML of the virtual machine:

So we need them as well if we want to use them on another computer.

To add the snapshot to the other computer, I have to run:

However, keep in mind that if you try to start a snapshot, you get this warning:

So if you don’t want to lose the current state, create another snapshot for the current state before restoring a previous one. Moreover, if the snapshot’s state is “Shutoff”, “starting” the snapshot only restores it. Then, you must start the virtual machine.

A different approach: symlinks

In the previous sections, I showed how machine information (including snapshots) and images could be put on external drives. Besides the machine images residing on external drives from the beginning, the machine metadata is still on your hard disk. In fact, you must first export them (e.g., on the external drive) and then import them on another computer.

A more radical approach consists of keeping the metadata on the external drive only and creating symlinks in each computer’s libvirt/qemu directories.

On the first computer, the XML files of machine information and snapshots have to be copied onto the external drive. IMPORTANT: don’t dump information as we did above; you need to copy the original XML files themselves. Dumping does not generate the exact XML files stored on the libvirt/qemu directories. In fact, as shown above, the dumped XML files must be imported with dedicated commands.

In my case, on the first computer, I run:

So, on the external drive, I end up with these contents:

On the same computer, I run the following commands (make sure the “libvirtd.service” is not running):

Now, I can start the “libvirtd.service” and the default network, and I make sure I can still access all my machines stored on the external drive, including all the machine information.

Of course, if you have never created virtual machines and want to start creating them on the external drive, it is enough to run the above commands. Then, start creating machines. Remember to select the external drive for the image location.

Then, on the other computers where I have already installed the same software for KVM, QEMU, etc., I first ensure the “libvirtd.service” is not running (in case stop it). Then, I connect my external drive and run the above commands (these will remove possible existing machines’ information, so be careful).

Of course, the above commands must be run only the first time.

Now, I can start the “libvirtd.service” and the default network, and I can access all my machines stored on the external drive, including all the machine information. Every modification (an image content or a machine configuration) will be stored on the external drive.

This approach works if you want to store ALL your machines on the external drive. You won’t have to keep the information in sync because they are stored in a single place.

If you need to keep some machines on your computers and others on different external drives, you must use the above-shown manual procedure for exporting and importing. It is then up to you to remember to re-export/re-import if you change a machine’s configuration or a snapshot.

Happy virtualization! 🙂

6 thoughts on “KVM Virtual Machine Manager and Virtual Machines on external drives

  1. Jim Abernathy

    I use kvm a lot and I appreciate the new to me information about exporting/importing and snapshots. I have a question about the need to edit unix_sock_group, etc. in /etc/libvirt/libvirtd.conf. Why do that? I run KVM on multiple distros and all I do is add my user to group ‘libvirt’. I’ve never had an issue with needing sudo to run virt-manager.

    The one thing I always do is create a network bridge using nmcli to allow any VM in KVM to start sshd and allow any user on my network to ssh into that VM with the right credentials.

    1. Lorenzo Bettini Post author

      If I remember correctly, that’s part of the standard instructions for Arch/EndeavourOS.

  2. David Burke

    Thanks for the article. Do you think instead of an external drive you could use google drive for the storage pool?

    1. Lorenzo Bettini Post author

      Hi. It depends on how you mount the Google drive, e.g., if it’s visible as a standard mount point. However, I doubt it would be usable: it would be extremely slow, I guess.

      1. David Burke

        Thanks for the reply. I think you could use rclone to mount it but your comment about the speed is probablyt true

        1. Lorenzo Bettini Post author

          Moreover, I think that cloud services in general are not able to handle COW and sparse files like the ones used by KVM/QEMU.
          I guess that if you use rclone to mount the Google drive to a local directory you can put a virtual machine there without any problem, but then you’ll have to wait ages for the corresponding file to be completely uploaded (and I guess you soon hit tranfer limits imposed by the cloud service)… I wouldn’t try 😉


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