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13.3. Using Storage Volumes

This section provides information about using storage volumes. It provides conceptual information, as well as detailed instructions on creating, configuring, and deleting storage volumes using virsh commands and the Virtual Machine Manager.

13.3.1. Storage Volume Concepts

Storage pools are divided into storage volumes. Storage volumes are abstractions of physical partitions, LVM logical volumes, file-based disk images, and other storage types handled by libvirt. Storage volumes are presented to guest virtual machines as local storage devices regardless of the underlying hardware.

Note

The sections below do not contain all of the possible commands and arguments that virsh provides for managing storage volumes> For more information, see Section 20.30, “Storage Volume Commands”.
On the host machine, a storage volume is referred to by its name and an identifier for the storage pool from which it derives. On the virsh command line, this takes the form --pool storage_pool volume_name.
For example, a volume named firstimage in the guest_images pool. 
# virsh vol-info --pool guest_images firstimage
  Name:           firstimage
  Type:           block
  Capacity:       20.00 GB
  Allocation:     20.00 GB
For additional parameters and arguments, see Section 20.34, “Listing Volume Information”.

13.3.2. Creating Storage Volumes

This section provides general instructions for creating storage volumes from storage pools using virsh and the Virtual Machine Manager. After creating storage volumes, you can add storage devices to guests.

13.3.2.1. Creating Storage Volumes with virsh

Do one of the following:
  • Define the storage volume using an XML file.
    a. Create a temporary XML file containing the storage volume information required for the new device.
    The XML file must contain specific fields including the following:
    • name - The name of the storage volume.
    • allocation - The total storage allocation for the storage volume.
    • capacity - The logical capacity of the storage volume. If the volume is sparse, this value can differ from the allocation value.
    • target - The path to the storage volume on the host system and optionally its permissions and label.
    The following shows an example a storage volume definition XML file. In this example, the file is saved to ~/guest_volume.xml 
      <volume>
    <name>volume1</name>
    <allocation>0</allocation>
    <capacity>20G</capacity>
    <target>
      <path>/var/lib/virt/images/sparse.img</path>
    </target>
  </volume>
    b. Use the virsh vol-create command to create the storage volume based on the XML file. 
    # virsh vol-create guest_images_dir ~/guest_volume.xml
  Vol volume1 created
    c. Delete the XML file created in step a.
  • Use the virsh vol-create-as command to create the storage volume. 
    # virsh vol-create-as guest_images_dir volume1 20GB --allocation 0
  • Clone an existing storage volume using the virsh vol-clone command. The virsh vol-clone command must specify the storage pool that contains the storage volume to clone and the name of the newly created storage volume. 
    # virsh vol-clone --pool guest_images_dir volume1 clone1

13.3.2.2. Creating storage volumes with Virtual Machine Manager

Procedure 13.11. Creating Storage Volumes with Virtual Machine Manager

  1. Open the storage settings

    1. In the Virtual Machine Manager, open the Edit menu and select Connection Details.
    2. Click the Storage tab in the Connection Details window.
      Storage tab

      Figure 13.11. Storage tab

      The pane on the left of the Connection Details window shows a list of storage pools.

  2. Select the storage pool in which you want to create a storage volume

    In the list of storage pools, click the storage pool in which you want to create the storage volume.
    Any storage volumes configured on the selected storage pool appear in the Volumes pane at the bottom of the window.

  3. Add a new storage volume

    Click the button above the Volumes list. The Add a Storage Volume dialog appears.
    Create storage volume

    Figure 13.12. Create storage volume

  4. Configure the storage volume

    Configure the storage volume with the following parameters:
    • Enter a name for the storage pool in the Name field.
    • Select a format for the storage volume from the Format list.
    • Enter the maximum size for the storage volume in the Max Capacity field.

  5. Finish the creation

    Click Finish. The Add a Storage Volume dialog closes, and the storage volume appears in the Volumes list.

13.3.3. Viewing Storage Volumes

You can create multiple storage volumes from a storage pool. You can also use the virsh vol-list command to list the storage volumes in a storage pool. In the following example, the guest_images_disk contains three volumes. 
# virsh vol-create-as guest_images_disk volume1 8G
Vol volume1 created

# virsh vol-create-as guest_images_disk volume2 8G
Vol volume2 created

# virsh vol-create-as guest_images_disk volume3 8G
Vol volume3 created

# virsh vol-list guest_images_disk
Name                 Path
-----------------------------------------
volume1              /home/VirtualMachines/guest_images_dir/volume1
volume2              /home/VirtualMachines/guest_images_dir/volume2
volume3              /home/VirtualMachines/guest_images_dir/volume3

13.3.4. Managing Data

This section provides information about managing the data on storage volumes.

Note

Some types of storage volumes do not support all of the data management commands. For specific information, see the sections below.

13.3.4.1. Wiping Storage Volumes

To ensure that data on a storage volume cannot be accessed, a storage volume can be wiped using the virsh vol-wipe command.
Use the virsh vol-wipe command to wipe a storage volume: 
# virsh vol-wipe new-vol vdisk
By default, the data is overwritten by zeroes. However, there are a number of different methods that can be specified for wiping the storage volume. For detailed information about all of the options for the virsh vol-wipe command, refer to Section 20.32, “Deleting a Storage Volume's Contents”.

13.3.4.2. Uploading Data to a Storage Volume

You can upload data from a specified local file to a storage volume using the virsh vol-upload command. 
# virsh vol-upload --pool pool-or-uuid --offset bytes --length bytes vol-name-or-key-or-path local-file
The following are the main virsh vol-upload options:
  • --pool pool-or-uuid - The name or UUID of the storage pool the volume is in.
  • vol-name-or-key-or-path - The name or key or path of the volume to upload.
  • --offset bytes The position in the storage volume at which to start writing the data.
  • --length length - An upper limit for the amount of data to be uploaded.

    Note

    An error will occur if local-file is greater than the specified --length.

Example 13.1. Uploading data to a storage volume

# virsh vol-upload sde1 /tmp/data500m.empty disk-pool
In this example sde1 is a volume in the disk-pool storage pool. The data in /tmp/data500m.empty is copied to sde1.

13.3.4.3. Downloading Data to a Storage Volume

You can download data from a storage volume to a specified local file using the virsh vol-download command. 
# vol-download --pool pool-or-uuid --offset bytes --length bytes vol-name-or-key-or-path local-file
The following are the main virsh vol-download options:
  • --pool pool-or-uuid - The name or UUID of the storage pool that the volume is in.
  • vol-name-or-key-or-path - The name, key, or path of the volume to download.
  • --offset - The position in the storage volume at which to start reading the data.
  • --length length - An upper limit for the amount of data to be downloaded.

Example 13.2. Downloading data from a storage volume

# virsh vol-download sde1 /tmp/data-sde1.tmp disk-pool
In this example sde1 is a volume in the disk-pool storage pool. The data in sde1 is downloaded to /tmp/data-sde1.tmp.

13.3.4.4. Resizing Storage Volumes

You can resize the capacity of a specified storage volume using the vol-resize command. 
# virsh vol-resize --pool pool-or-uuid vol-name-or-path pool-or-uuid capacity --allocate --delta --shrink
The capacity is expressed in bytes. The command requires --pool pool-or-uuid which is the name or UUID of the storage pool the volume is in. This command also requires vol-name-or-key-or-path, the name, key, or path of the volume to resize.
The new capacity might be sparse unless --allocate is specified. Normally, capacity is the new size, but if --delta is present, then it is added to the existing size. Attempts to shrink the volume will fail unless --shrink is present.
Note that capacity cannot be negative unless --shrink is provided and a negative sign is not necessary. capacity is a scaled integer which defaults to bytes if there is no suffix. In addition, note that this command is only safe for storage volumes not in use by an active guest. Refer to Section 20.13.3, “Changing the Size of a Guest Virtual Machine's Block Device” for live resizing.

Example 13.3. Resizing a storage volume

For example, if you created a 50M storage volume, you can resize it to 100M with the following command: 
# virsh vol-resize --pool disk-pool sde1 100M

13.3.5. Deleting Storage Volumes

You can delete storage volumes using virsh or the Virtual Machine Manager.

Note

To avoid negatively affecting guest virtual machines that use the storage volume you want to delete, it is recommended that you release any resources using it.

13.3.5.1. Deleting storage volumes using virsh

Delete a storage volume using the virsh vol-delete command. The command must specify the name or path of the storage volume and the storage pool from which the storage volume is abstracted.
The following example deletes the volume_name storage volume from the guest_images_dir storage pool: 
# virsh vol-delete volume_name --pool guest_images_dir
  vol volume_name deleted

13.3.5.2. Deleting storage volumes using Virtual Machine Manager

Procedure 13.12. Deleting Storage Volumes with Virtual Machine Manager

  1. Open the storage settings

    1. In the Virtual Machine Manager, open the Edit menu and select Connection Details.
    2. Click the Storage tab in the Connection Details window.
      Storage tab

      Figure 13.13. Storage tab

      The pane on the left of the Connection Details window shows a list of storage pools.

  2. Select the storage volume you want to delete

    1. In the list of storage pools, click the storage pool from which the storage volume is abstracted.
      A list of storage volumes configured on the selected storage pool appear in the Volumes pane at the bottom of the window.
    2. Select the storage volume you want to delete.

  3. Delete the storage volume

    1. Click the button (above the Volumes list). A confirmation dialog appears.
    2. Click Yes. The selected storage volume is deleted.

13.3.6. Adding Storage Devices to Guests

You can add storage devices to guest virtual machines using virsh or Virtual Machine Manager.

13.3.6.1. Adding Storage Devices to Guests Using virsh

To add storage devices to a guest virtual machine, use the attach-disk command. The arguments that contain information about the disk to add can be specified in an XML file or on the command line.
The following is a sample XML file with the definition of the storage. 
<disk type='file' device='disk>'>
  <driver name='qemu' type='raw' cache='none'/>
  <source file='/var/lib/libvirt/images/FileName.img'/>
  <target dev='vdb' bus='virtio'/>
</disk>
The following command attaches a disk to Guest1 using an XML file called NewStorage.xml. 
# virsh attach-disk --config Guest1 ~/NewStorage.xml
The following command attaches a disk to Guest1 without using an xml file. 
# virsh attach-disk --config Guest1 --source /var/lib/libvirt/images/FileName.img --target vdb

13.3.6.2. Adding Storage Devices to Guests Using Virtual Machine Manager

You can add a storage volume to a guest virtual machine or create and add a default storage device to a guest virtual machine.
13.3.6.2.1. Adding a storage volume to a guest
To add a storage volume to a guest virtual machine:

  1. Open Virtual Machine Manager to the virtual machine hardware details window

    Open virt-manager by executing the virt-manager command as root or opening Applications System Tools Virtual Machine Manager.
    The Virtual Machine Manager window

    Figure 13.14. The Virtual Machine Manager window

    Select the guest virtual machine to which you want to add a storage volume.
    Click Open. The Virtual Machine window opens.
    Click . The hardware details window appears.
    The Hardware Details window

    Figure 13.15. The Hardware Details window

  2. Open the Add New Virtual Hardware window

    Click Add Hardware. The Add New Virtual Hardware window appears.
    Ensure that Storage is selected in the hardware type pane.
    The Add New Virtual Hardware window

    Figure 13.16. The Add New Virtual Hardware window

  3. View a list of storage volumes

    Select the Select or create custom storage option button.
    Click Manage. The Choose Storage Volume dialog appears.
    The Select Storage Volume window

    Figure 13.17. The Select Storage Volume window

  4. Select a storage volume

    Select a storage pool from the list on the left side of the Select Storage Volume window. A list of storage volumes in the selected storage pool appears in the Volumes list.

    Note

    You can create a storage pool from the Select Storage Volume window. For more information, see Section 13.2.2.2, “Creating storage pools with Virtual Machine Manager”.
    Select a storage volume from the Volumes list.

    Note

    You can create a storage volume from the Select Storage Volume window. For more information, see Section 13.3.2.2, “Creating storage volumes with Virtual Machine Manager”.
    Click Choose Volume. The Select Storage Volume window closes.

  5. Configure the storage volume

    Select a device type from the Device type list. Available types are: Disk device, Floppy device, and LUN Passthrough.
    Select a bus type from the Bus type list. The available bus types are dependent on the selected device type.
    Select a cache mode from the Cache mode list. Available cache modes are: Hypervisor default, none, writethrough, writeback, directsync, unsafe
    Click Finish. The Add New Virtual Hardware window closes.
13.3.6.2.2. Adding default storage to a guest
The default storage pool is a file-based image in /var/lib/libvirt/images/ directory.
To add default storage to a guest virtual machine:

  1. Open Virtual Machine Manager to the virtual machine hardware details window

    Open virt-manager by executing the virt-manager command as root or opening Applications System Tools Virtual Machine Manager.
    The Virtual Machine Manager window

    Figure 13.18. The Virtual Machine Manager window

    Select the guest virtual machine to which you want to add a storage volume.
    Click Open. The Virtual Machine window opens.
    Click . The hardware details window appears.
    The Hardware Details window

    Figure 13.19. The Hardware Details window

  2. Open the Add New Virtual Hardware window

    Click Add Hardware. The Add New Virtual Hardware window appears.
    Ensure that Storage is selected in the hardware type pane.
    The Add New Virtual Hardware window

    Figure 13.20. The Add New Virtual Hardware window

  3. Create a disk for the guest

    Ensure that the Create a disk image for the virtual machine option.
    Enter the size of the disk to create in the textbox below the Create a disk image for the virtual machine option button.
    Click Finish. The Add New Virtual Hardware window closes.

13.3.6.3. Managing Storage Controllers in a Guest Virtual Machine

Unlike virtio disks, SCSI devices require the presence of a controller in the guest virtual machine. This section details the necessary steps to create a virtual SCSI controller (also known as "Host Bus Adapter", or HBA), and to add SCSI storage to the guest virtual machine.

Procedure 13.13. Creating a virtual SCSI controller

  1. Display the configuration of the guest virtual machine (Guest1) and look for a pre-existing SCSI controller: 
    # virsh dumpxml Guest1 | grep controller.*scsi
    If a device controller is present, the command will output one or more lines similar to the following: 
    <controller type='scsi' model='virtio-scsi' index='0'/>
  2. If the previous step did not show a device controller, create the description for one in a new file and add it to the virtual machine, using the following steps:
    1. Create the device controller by writing a <controller> element in a new file and save this file with an XML extension. virtio-scsi-controller.xml, for example. 
      <controller type='scsi' model='virtio-scsi'/>
    2. Associate the device controller you just created in virtio-scsi-controller.xml with your guest virtual machine (Guest1, for example): 
      # virsh attach-device --config Guest1 ~/virtio-scsi-controller.xml
      In this example the --config option behaves the same as it does for disks. See Section 13.3.6, “Adding Storage Devices to Guests” for more information.
  3. Add a new SCSI disk or CD-ROM. The new disk can be added using the methods in Section 13.3.6, “Adding Storage Devices to Guests”. In order to create a SCSI disk, specify a target device name that starts with sd.

    Note

    The supported limit for each controller is 1024 virtio-scsi disks, but it is possible that other available resources in the host (such as file descriptors) are exhausted with fewer disks.
    For more information, see the following Red Hat Enterprise Linux 6 whitepaper: The next-generation storage interface for the Red Hat Enterprise Linux Kernel Virtual Machine: virtio-scsi. 
    # virsh attach-disk Guest1 /var/lib/libvirt/images/FileName.img sdb --cache none
    Depending on the version of the driver in the guest virtual machine, the new disk may not be detected immediately by a running guest virtual machine. Follow the steps in the Red Hat Enterprise Linux Storage Administration Guide.

13.3.7. Removing Storage Devices from Guests

You can remove storage device from virtual guest machines using virsh or Virtual Machine Manager.

13.3.7.1. Removing Storage from a Virtual Machine with virsh

The following example removes the vdb storage volume from the Guest1 virtual machine: 
# virsh detach-disk Guest1 vdb

13.3.7.2. Removing Storage from a Virtual Machine with Virtual Machine Manager

Procedure 13.14. Removing storage from a virtual machine with Virtual Machine Manager

To remove storage from a guest virtual machine using Virtual Machine Manager:

  1. Open Virtual Machine Manager to the virtual machine hardware details window

    Open virt-manager by executing the virt-manager command as root or opening Applications System Tools Virtual Machine Manager.
    Select the guest virtual machine from which you want to remove a storage device.
    Click Open. The Virtual Machine window opens.
    Click . The hardware details window appears.

  2. Remove the storage from the guest virtual machine

    Select the storage device from the list of hardware on the left side of the hardware details pane.
    Click Remove. A confirmation dialog appears.
    Click Yes. The storage is removed from the guest virtual machine.
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