Creating and mounting a fileshare as a file in a KVM Windows guest

Yeah, the title is a mouthfull.

So, let's get it done.

1. Create the disk, which is henceforth called foretest.iso. I made it to be 1G because, as the name says, it is a test.

   raub@vmhost:~$ dd if=/dev/zero of=foretest.iso bs=1G count=1
   1+0 records in
   1+0 records out
   1073741824 bytes (1.1 GB, 1.0 GiB) copied, 2.29044 s, 469 MB/s
   raub@vmhost:~$

   NOTE: Some people would complain that I am using the .iso extension for my image file instead of, say, .img. Well, this is my article; deal with it.

2. Now let's feed it to the vm.

   raub@vmhost:~$ virsh attach-disk desktop dev/foretest.iso hda --type raw --mode readwrite                 
   error: No support for readwrite in command 'attach-disk'
   
   raub@vmhost:~$

   Well, I did not know my KVM host does not have attach-disk. I guess I need to use attach-device instead. But, we will need to use a config file to describe how we want the storage to look like. If it looks familiar, we have used attach-disk to mount a USB device, namely an APC UPS.

   So, here is the .xml file:

   cat > dev/foretest.xml << 'EOF'
   <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source file='/home/raub/dev/foretest.iso'/>
   <target dev='hdc'/>
   </disk >
   EOF

   And here we are feeding it into the guest:

   raub@vmhost:~$ sudo virsh attach-device --config testdesktop dev/foretest.xml
   Device attached successfully
   
   raub@vmhost:~$

   So we should be able to go to the guest and see the drive waving at us, right? Er, not quite. it is being listed here

   <disk type='file' device='disk'>
   
         <source file='/home/raub/dev/foretest.iso'/>
         <target dev='vdd' bus='virtio'/>
         <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
      </disk>

   But not in the GUI thingie nor inside the guest. And, yes, I have 2 virtual CD drives in this guest.

   
   

On a second thought, maybe I am doing this wrong. You see, the --config option tells it to add the drive to the config file. Only way to enable that is to completely shut down the vm guest (rebooting is not enough) and then restart it. What if I use --live instead, which does the deed in real time, as if I just pop the computer on and add the drive to an unused sata port?

raub@vmhost:~$ sudo virsh attach-device --live testdesktop dev/foretest.xml
error: Failed to attach device from dev/foretest.xml
error: internal error unable to execute QEMU command '__com.redhat_drive_add': Device 'drive-virtio-disk3' could not be initialized

raub@vmhost:~$

I guess it does not like me to add hard drives to a running guest. We need to try something else.

Attempt #2: USB

What if we pretend our disk is really a removable device like a USB drive? We begin by rewriting our .xml file:

cat > dev/foretest.xml << 'EOF'
<disk type='file' device='disk'>
  <driver name='qemu' type='raw'/>
  <source file='/home/raub/dev/foretest.iso'/>
  <target dev='sdd' bus='usb'/>
</disk>
EOF

Now let's mount it:

raub@desktop:~$ sudo virsh attach-device --live testdesktop dev/foretest.xml
Device attached successfully

raub@desktop:~$

So far so good. Can our guest see it this time? When I use Windows Explorer (do notice I disabled one of the CD drives as I was doing something else between the last screen capture and this one) I do not see the drive there; I kinda expected Windows to say something like "Hey! You just connected an unformatted drive! I need to format it!" Oh well; there are more than one way to get this done:

There it is! And we can format it!

And then, we can put things into it.

Now, let's unmount it

raub@desktop:~$ sudo virsh detach-device --live testdesktop dev/foretest.xml
Device detached successfully

raub@desktop:~$

Now let's make it a physical drive!

Stupid question: since we pretend our foretest.iso file is a USB drive, can we make it a real USB drive? We would test that question by first grabbing a USB drive which is at least 1GB in size (I had a 16GB one doing nothing), which was mounted into my KVM host as /dev/sdh.And now we need to copy the file to the drive.

raub@desktop:~$ sudo dd if=dev/foretest.iso of=/dev/sdh
raub@desktop:~$

Then grab USB drive and then mount it in a windows physical desktop. And it will look just like how it looks in the testdesktop guest down to be able to write to it.

The next step will be to see if we can make it a system disk, a disk we can boot a computer from. But, that will be for another article.