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9.5. Managing Storage Controllers in a Guest Virtual Machine

Starting from Red Hat Enterprise Linux 6.4, it is supported to add SCSI and virtio-SCSI devices to guest virtual machines that are running Red Hat Enterprise Linux 6.4 or later. Unlike virtio disks, SCSI devices require the presence of a controller in the guest virtual machine. Virtio-SCSI provides the ability to connect directly to SCSI LUNs and significantly improves scalability compared to virtio-blk. The advantage of virtio-SCSI is that it is capable of handling hundreds of devices compared to virtio-blk which can only handle 28 devices and exhausts PCI slots. Virtio-SCSI is now capable of inheriting the feature set of the target device with the ability to:
  • attach a virtual hard drive or CD through the virtio-scsi controller,
  • pass-through a physical SCSI device from the host to the guest via the QEMU scsi-block device,
  • and allow the usage of hundreds of devices per guest; an improvement from the 28-device limit of virtio-blk.
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 9.10. 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. Refer to Procedure 13.2, “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 13.3.1, “Adding File-based Storage to a Guest” and Section 13.3.2, “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.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.