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Chapter 20. Sharing files between the host and its virtual machines

You may frequently require to share data between your host system and the virtual machines (VMs) it runs. To do so quickly and efficiently, you can set up NFS file shares on your system. Alternatively, you can also use the virtiofs to share data with your Linux and Windows VMs.

20.1. Sharing files between the host and its virtual machines by using NFS

For efficient file sharing between the RHEL 9 host system and the virtual machines (VMs), you can export an NFS share that VMs can mount and access.

However with Linux VMs, it is usually more convenient to use the virtiofs feature.

Prerequisites

  • The nfs-utils package is installed on the host. 

    # dnf install nfs-utils -y
  • Virtual network of NAT or bridge type is configured to connect a host to VMs.
  • Optional: For improved security, ensure your VMs are compatible with NFS version 4 or later.

Procedure

  1. On the host, export a directory with the files you want to share as a network file system (NFS):

    1. Share an existing directory with VMs. If you do not want to share any of the existing directories, create a new one: 

      # mkdir shared-files

    2. Obtain the IP address of each VM to share files from the host, for example, testguest1 and testguest2 : 

      # virsh domifaddr testguest1
Name       MAC address          Protocol     Address
----------------------------------------------------------------
vnet0      52:53:00:84:57:90    ipv4         192.0.2.2/24

# virsh domifaddr testguest2
Name       MAC address          Protocol     Address
----------------------------------------------------------------
vnet1      52:53:00:65:29:21    ipv4         192.0.2.3/24

    3. Edit the /etc/exports file on the host and add a line that includes the directory you want to share, IPs of VMs to share, and additional options: 

      /home/<username>/Downloads/<shared_directory>/ <VM1-IP(options)> <VM2-IP(options)>
...

      The following example shares the /usr/local/shared-files directory on the host with testguest1 and testguest2, and enables the VMs to edit the content of the directory: 

      /usr/local/shared-files/ 192.0.2.2(rw,sync) 192.0.2.3(rw,sync)
      Note

      To share a directory with a Windows VM, you need to ensure the Windows NFS client has write permissions in the shared directory. You can use the all_squash, anonuid, and anongid options in the /etc/exports file.

      /usr/local/shared-files/ 192.0.2.2(rw,sync,all_squash,anonuid=<directory-owner-UID>,anongid=<directory-owner-GID>)

      The <directory-owner-UID> and <directory-owner-GID> are the UID and GID of the local user that owns the shared directory on the host.

      For other options to manage NFS client permissions, follow the Securing the NFS service guide.

    4. Export the updated file system: 

      # exportfs -a

    5. Start the nfs-server service: 

      # systemctl start nfs-server

    6. Obtain the IP address of the host system to mount the shared directory on the VMs: 

      # ip addr
...
5: virbr0: [BROADCAST,MULTICAST,UP,LOWER_UP] mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 52:54:00:32:ff:a5 brd ff:ff:ff:ff:ff:ff
inet 192.0.2.1/24 brd 192.0.2.255 scope global virbr0
valid_lft forever preferred_lft forever
...

      Note that the relevant network connects the host with VMs to share files. Usually, this is virbr0.

  2. Mount the shared directory on a Linux VM that is specified in the /etc/exports file: 

    # mount 192.0.2.1:/usr/local/shared-files /mnt/host-share
    • 192.0.2.1: The IP address of the host.
    • /usr/local/shared-files: A file-system path to the exported directory on the host.

    • /mnt/host-share: A mount point on the VM

      Note

      The mount point must be an empty directory.

  3. To mount the shared directory on a Windows VM as mentioned in the /etc/exports file:

    1. Open a PowerShell shell prompt as an Administrator.

    2. Install the NFS-Client package on the Windows.

      1. To install on a server version, enter: 

        # Install-WindowsFeature NFS-Client

      2. To install on a desktop version, enter: 

        # Enable-WindowsOptionalFeature -FeatureName ServicesForNFS-ClientOnly, ClientForNFS-Infrastructure -Online -NoRestart

    3. Mount the directory exported by the host on a Windows VM: 

      # C:\Windows\system32\mount.exe -o anon \\192.0.2.1\usr\local\shared-files Z:

      In this example:

      • 192.0.2.1: The IP address of the host.
      • /usr/local/shared-files: A file system path to the exported directory on the host.

      • Z:: The drive letter for a mount point.

        Note

        You must choose a drive letter that is not in use on the system.

Verification

  • List the contents of the shared directory on the VM so that you can share files between the host and the VM: 

    $ ls <mount_point>
shared-file1  shared-file2  shared-file3

    In this example, replace <mount_point> with a file system path to the mounted shared directory.

Additional resources

20.2. Sharing files between the host and its virtual machines by using virtiofs

Using virtiofs, you can share files between your host and your virtual machines (VM) as a directory tree that works the same as the local file system structure.

20.2.1. Sharing files between the host and its virtual machines by using virtiofs

When using RHEL 9 as your hypervisor, you can efficiently share files between your host system and its virtual machines (VM) using the virtiofs feature.

Prerequisites

  • Virtualization is installed and enabled on your RHEL 9 host.

  • A directory that you want to share with your VMs. If you do not want to share any of your existing directories, create a new one, for example named shared-files. 

    # mkdir /root/shared-files
  • The VM you want to share data with is using a Linux distribution as its guest operating system.

Procedure

  1. For each directory on the host that you want to share with your VM, set it as a virtiofs file system in the VM’s XML configuration.

    1. Open the XML configuration of the intended VM. 

      # virsh edit vm-name

    2. Add an entry similar to the following to the <devices> section of the VM’s XML configuration. 

      <filesystem type='mount' accessmode='passthrough'>
  <driver type='virtiofs'/>
  <binary path='/usr/libexec/virtiofsd' xattr='on'/>
  <source dir='/root/shared-files'/>
  <target dir='host-file-share'/>
</filesystem>

      This example sets the /root/shared-files directory on the host to be visible as host-file-share to the VM.

  2. Set up shared memory for the VM. To do so, add shared memory backing to the <domain> section of the XML configuration: 

    <domain>
 [...]
 <memoryBacking>
   <access mode='shared'/>
 </memoryBacking>
 [...]
</domain>

  3. Boot up the VM. 

    # virsh start vm-name

  4. Mount the file system in the guest operating system. The following example mounts the previously configured host-file-share directory with a Linux guest operating system. 

    # mount -t virtiofs host-file-share /mnt

Verification

  • Ensure that the shared directory became accessible on the VM and that you can now open files stored in the directory.

Known issues and limitations

  • File-system mount options related to access time, such as noatime and strictatime, are not likely to work with virtiofs, and Red Hat discourages their use.

Troubleshooting

  • If virtiofs is not optimal for your usecase or supported for your system, you can use NFS instead.

20.2.2. Sharing files between the host and Windows virtual machines by using virtiofs

When using RHEL 9 as your hypervisor, you can efficiently share files between your host system and Windows virtual machines (VM) using the virtiofs feature along with the virtio-win package.

Note

You can run the virtiofs service in case insensitive mode on a Windows VM using the virtiofs.exe command and the -i parameter.

Prerequisites

Procedure

  1. On your Windows VM, install WinFsp. To do so, mount the virtio-win ISO image, start the winfsp MSI installer, and follow the prompts.

    In the Custom Setup window of the installation wizard, select the features you want to install on the VM.

  2. Start the virtiofs service: 

    # sc start VirtioFsSvc

  3. Navigate to This PC:

    File Explorer This PC

    Virtiofs should be available on the Windows VM as the first available drive letter starting with z: and going backwards. For example, my_viofs (Z:).

    Important

    You must restart the virtiofs service after each VM reboot to access the shared directory.

  4. Optional: To set up additional virtiofs instances:

    1. Stop the virtiofs service: 

      # sc stop VirtioFsSvc
# sc config VirtioFsSvc start=demand

    2. Configure the WinFSP.Launcher service to set up multiple virtiofs instances: 

      # "C:\Program Files (x86)\WinFsp\bin\fsreg.bat" virtiofs "<path to the binary>\virtiofs.exe" "-t %1 -m %2"

    3. Mount virtiofs instances to drives.

      For example, to mount virtiofs with the tag mount_tag0 to the Y: drive: 

      "C:\Program Files (x86)\WinFsp\bin\launchctl-x64.exe" start virtiofs viofsY mount_tag0 Y:
    4. Repeat the previous step to mount all of your virtiofs instances.

    5. To unmount the virtiofs instance: 

      "C:\Program Files (x86)\WinFsp\bin\launchctl-x64.exe" stop virtiofs viofsY

Verification

  1. On your Windows VM, navigate to This PC:

    File Explorer This PC

    • If you did not specify a mount point when setting up the virtiofs service, it will use the first available drive letter starting with z: and going backwards.
    • If you have multiple virtiofs instances set up, they will appear as drives with the letters you had assigned to the instances.

20.2.3. Sharing files between the host and its virtual machines by using virtiofs in the web console

You can use the RHEL web console to efficiently share files between your host system and its virtual machines (VM) using the virtiofs feature.

Prerequisites

  • The web console VM plug-in is installed on your system.

  • A directory that you want to share with your VMs. If you do not want to share any of your existing directories, create a new one, for example named centurion. 

    # mkdir /home/centurion
  • The VM you want to share data with is using a Linux distribution as its guest operating system.

Procedure

  1. In the Virtual Machines interface, click the VM with which you want to share files.

    A new page opens with an Overview section with basic information about the selected VM and a Console section.

  2. Scroll to Shared directories.

    The Shared directories section displays information about the host files and directories shared with that VM and options to Add or Remove a shared directory.

    Image displaying the directories shared with the selected VM.

  3. Click Add shared directory.

    The Share a host directory with the guest dialog appears.

    Image displaying the Share a host directory with the guest dialog box.

  4. Enter the following information:

    • Source path - The path to the host directory that you want to share.
    • Mount tag - The tag that the VM uses to mount the directory.

  5. Set additional options:

    • Extended attributes - Set whether to enable extended attributes, xattr, on the shared files and directories.

  6. Click Share.

    The selected directory is shared with the VM.

Verification

  • Ensure that the shared directory is accessible on the VM and you can now open files stored in that directory.

20.2.4. Using the web console to remove shared files between the host and its virtual machines by using virtiofs

You can use the RHEL web console to remove files shared between your host system and its virtual machines (VM) using the virtiofs feature.

Prerequisites

Procedure

  1. In the Virtual Machines interface, click on the VM from which you want to remove the shared files.

    A new page opens with an Overview section with basic information about the selected VM and a Console section.

  2. Scroll to Shared directories.

    The Shared directories section displays information about the host files and directories shared with that VM and options to Add or Remove a shared directory.

    Image displaying the directories shared with the selected VM.

  3. Click Remove next to the directory that you want to unshare with the VM.

    The Remove filesystem dialog appears.

    Image displaying the remove a filesystem dialog box.

  4. Click Remove.

    The selected directory is unshared with the VM.

Verification

  • The shared directory is no longer available and accessible on the VM.
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