Procedure 12.1. Adding file-based storage

1. Create a storage file or use an existing file (such as an ISO file). Note that both of the following commands create a 4GB file which can be used as additional storage for a guest:
   • Pre-allocated files are recommended for file-based storage images. Create a pre-allocated file using the following dd command as shown:

     # dd if=/dev/zero of=/var/lib/libvirt/images/FileName.iso bs=1M count=4096

   • Alternatively, create a sparse file instead of a pre-allocated file. Sparse files are created much faster and can be used for testing, but are not recommended for production environments due to data integrity and performance issues.

     # dd if=/dev/zero of=/var/lib/libvirt/images/FileName.iso bs=1M seek=4096 count=0

2. Create the additional storage by writing a <disk> element in a new file. In this example, this file will be known as NewStorage.xml.
   A <disk> element describes the source of the disk, and a device name for the virtual block device. The device name should be unique across all devices in the guest, and identifies the bus on which the guest will find the virtual block device. The following example defines a virtio block device whose source is a file-based storage container named FileName.img:

   <disk type='file' device='disk'>   <driver name='qemu' type='raw' cache='none'/>   <source file='/var/lib/libvirt/images/FileName.img'/>   <target dev='vdb'/></disk>

   Device names can also start with "hd" or "sd", identifying respectively an IDE and a SCSI disk. The configuration file can also contain an <address> sub-element that specifies the position on the bus for the new device. In the case of virtio block devices, this should be a PCI address. Omitting the <address> sub-element lets libvirt locate and assign the next available PCI slot.
3. Attach the CD-ROM as follows:

   <disk type='file' device='cdrom'>   <driver name='qemu' type='raw' cache='none'/>   <source file='/var/lib/libvirt/images/FileName.iso'/>   <readonly/>   <target dev='hdc'/></disk >

4. Add the device defined in NewStorage.xml with your guest (Guest1):

   # virsh attach-device --config Guest1 ~/NewStorage.xml

   Note

   This change will only apply after the guest has been destroyed and restarted. In addition, persistent devices can only be added to a persistent domain, that is a domain whose configuration has been saved with virsh define command.
   If the guest is running, and you want the new device to be added temporarily until the guest is destroyed, omit the --config option:

   # virsh attach-device Guest1 ~/NewStorage.xml

   Note

   The virsh command allows for an attach-disk command that can set a limited number of parameters with a simpler syntax and without the need to create an XML file. The attach-disk command is used in a similar manner to the attach-device command mentioned previously, as shown:

   # virsh attach-disk Guest1 /var/lib/libvirt/images/FileName.iso vdb --cache none

   Note that the virsh attach-disk command also accepts the --config option.
5. Start the guest machine (if it is currently not running):

   # virsh start Guest1

   Note

   The following steps are Linux guest specific. Other operating systems handle new storage devices in different ways. For other systems, refer to that operating system's documentation.

6. Partitioning the disk drive

   The guest now has a hard disk device called /dev/vdb. If required, partition this disk drive and format the partitions. If you do not see the device that you added, then it indicates that there is an issue with the disk hotplug in your guest's operating system.
   1. Start fdisk for the new device:

      # fdisk /dev/vdbCommand (m for help):

   2. Type n for a new partition.
   3. The following appears:

      Command actione   extendedp   primary partition (1-4)

      Type p for a primary partition.
   4. Choose an available partition number. In this example, the first partition is chosen by entering 1.

      Partition number (1-4): 1

   5. Enter the default first cylinder by pressing Enter.

      First cylinder (1-400, default 1):

   6. Select the size of the partition. In this example the entire disk is allocated by pressing Enter.

      Last cylinder or +size or +sizeM or +sizeK (2-400, default 400):

   7. Enter t to configure the partition type.

      Command (m for help): t

   8. Select the partition you created in the previous steps. In this example, the partition number is 1 as there was only one partition created and fdisk automatically selected partition 1.

      Partition number (1-4): 1

   9. Enter 83 for a Linux partition.

      Hex code (type L to list codes): 83

   10. Enter w to write changes and quit.

       Command (m for help): w

   11. Format the new partition with the ext3 file system.

       # mke2fs -j /dev/vdb1

7. Create a mount directory, and mount the disk on the guest. In this example, the directory is located in myfiles.

   # mkdir /myfiles# mount /dev/vdb1 /myfiles

   The guest now has an additional virtualized file-based storage device. Note however, that this storage will not mount persistently across reboot unless defined in the guest's /etc/fstab file:

   /dev/vdb1 /myfiles ext3 defaults 0 0

Procedure 12.2. Adding physical block devices to guests

1. This procedure describes how to add a hard drive on the host to a guest. It applies to all physical block devices, including CD-ROM, DVD and floppy devices.
   Physically attach the hard disk device to the host. Configure the host if the drive is not accessible by default.
2. Do one of the following:
   1. Create the additional storage by writing a disk element in a new file. In this example, this file will be known as NewStorage.xml. The following example is a configuration file section which contains an additional device-based storage container for the host partition /dev/sr0:

      <disk type='block' device='disk'>  <driver name='qemu' type='raw' cache='none'/>  <source dev='/dev/sr0'/>  <target dev='vdc' bus='virtio'/></disk>

   2. Follow the instruction in the previous section to attach the device to the guest. Alternatively, you can use the virsh attach-disk command, as shown:

      # virsh attach-disk Guest1 /dev/sr0 vdc

      Note that the following options are available:
      • Thevirsh attach-disk command also accepts the --config, --type, and --mode options, as shown:

        # virsh attach-disk Guest1 /dev/sr0 vdc --config --type cdrom --mode readonly

      • Additionally, --type also accepts --type disk in cases where the device is a hard drive.
3. The guest now has a new hard disk device called /dev/vdc on Linux (or something similar, depending on what the guest OS chooses) or D: drive (for example) on Windows. You can now initialize the disk from the guest, following the standard procedures for the guest's operating system. Refer to Procedure 12.1, "Adding file-based storage" and Step 6 for an example.

   Warning

   The host should not use filesystem labels to identify file systems in the fstab file, the initrd file or on the kernel command line. Doing so presents a security risk if less privileged users, such as guests, have write access to whole partitions or LVM volumes, because a guest could potentially write a filesystem label belonging to the host, to its own block device storage. Upon reboot of the host, the host could then mistakenly use the guest's disk as a system disk, which would compromise the host system.

   It is preferable to use the UUID of a device to identify it in the fstab file, the initrd file or on the kernel command line. While using UUIDs is still not completely secure on certain file systems, a similar compromise with UUID is significantly less feasible.

   Important

   Guests should not be given write access to whole disks or block devices (for example, /dev/sdb). Guests with access to whole block devices may be able to modify volume labels, which can be used to compromise the host system. Use partitions (for example, /dev/sdb1) or LVM volumes to prevent this issue.

Procedure 12.3. Creating a virtual SCSI controller

1. Display the configuration of the guest (Guest1) and look for a pre-existing SCSI controller:

   # virsh dumpxml Guest1 | grep controller.*scsi

   If a 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 controller, create the description for one in a new file and add it to the virtual machine, using the following steps:
   1. Create the controller by writing a <controller> element in a new file and save this file with an XML extension. NewHBA.xml, for example.

      <controller type='scsi' model='virtio-scsi'/>

   2. Associate the device in the NewHBA.xml you just created with your guest:

      # virsh attach-device --config Guest1 ~/NewHBA.xml

      In this example the --config option behaves the same as it does for disks. Refer to Procedure 12.2, "Adding physical block 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 sections Section 12.3.1, "Adding file based storage to a guest" and Section 12.3.2, "Adding hard drives and other block devices to a guest". In order to create a SCSI disk, specify a target device name that starts with sd.

   # virsh attach-disk Guest1 /var/lib/libvirt/images/FileName.iso sdb --cache none

   Depending on the version of the driver in the guest, the new disk may not be detected immediately by a running guest. Follow the steps in the Red Hat Enterprise Linux Storage Administration Guide.