26장. Virtualization

If a virtio device is created where the number of vectors is set to a value higher than 32, the device behaves as if it was set to a zero value on Red Hat Enterprise Linux 6, but not on Enterprise Linux 7. The resulting vector setting mismatch causes a migration error if the number of vectors on any virtio device on either platform is set to 33 or higher. It is, therefore, not recommended to set the vector value to be greater than 32.

SELinux prevents qemu from attaching TUN/TAP queues. A multi-queue network interface controller (NIC) is disabled by default in Red Hat Enterprise Linux 7 and should not be turned on.

It is possible to add a Red Hat Enterprise Virtualization Manager 3.2 instance as a source in the Boxes tool: the list of virtual machines is displayed, but attempting to connect to a virtual machine can fail with a Connection failed message.

Live migration from Red Hat Enterprise Linux 6.5 to Red Hat Enterprise Linux 7 or from Red Hat Enterprise Linux 7 to Red Hat Enterprise Linux 7 fails if a SPICE client is connected to the virtual machine.

The thin_repair(8) manual page incorrectly describes that the thin_repair utility can repair metadata from an input file. However, this way of using thin_repair is not supported and the utility works properly only with metadata from devices. In order to repair metadata from a file, use the thin_restore utility instead.

The current implementation of the QEMU Guest Agent does not allow the Volume Shadow Copy Service (VSS) provider to create shadow copies within a guest. An attempt to create a shadow copy causes an error to be returned. This implementation should only be used as a means to freeze the file system.

When logging out from a session using QXL or SPICE on a virtual machine, it is impossible to return to GNOME Display Manager (GDM). Also, GDM cannot be restarted using the service gdm restart command.

The minimum VRAM size for a QXL device is 16MB in Red Hat Enterprise Linux 6, but it is 4KB in Red Hat Enterprise Linux 7. Consequently, specifying a VRAM size that is less than or equal to 8MB causes the actual VRAM size to differ between the versions, and live migration from Red Hat Enterprise Linux 6 to Red Hat Enterprise Linux 7 fails. To work around this problem, do not specify VRAM sizes that are less than or equal to 8MB on Red Hat Enterprise Linux 6. The default size of 9MB does not cause the described problem.

The serial property for QEMU's -drive option is deprecated in Red Hat Enterprise Linux 7. The serial number for virtual disks can still be specified using libvirt or QEMU's -device option.

Machine types providing the same virtual hardware as Red Hat Enterprise Linux 5 (for example, rhel-5.4.0) are no longer available in Red Hat Enterprise Linux 7. Therefore, virtual machines using these machine types cannot be started on or migrated to Red Hat Enterprise Linux 7 without first upgrading them to a newer machine type (for example, rhel-6.5.0 or pc).

The xrdb utility manages the X server resource database. Traditionally, the utility invokes the C preprocessor, which is provided by the cpp package, to expand resource files before further operations. However, this feature is disabled by default in Red Hat Enterprise Linux 7 because in most cases, the C preprocessor is not needed. To preserve previous behavior, install cpp and execute xrbd with the -cpp option.

The macvtap driver bypasses the standard Linux input stack and diverts the packets sent from user space. This also bypasses the mechanism for delivering packets to the tcpdump application. Consequently, the traffic on macvtap devices is not captured by default. To work around this problem, execute the following command:

tcpdump -i eth0 ether host mac_address

With the above command, tcpdump filters traffic by MAC address assigned to the macvtap device. As a result, traffic on macvtap devices is captured properly.

Under certain circumstances, migrating from Red Hat Enterprise Linux 6 to a Red Hat Enterprise Linux 7 host fails with the machine type rhel6.3.0 or earlier if a virtio-scsi device is present. This problem has been observed only with a Window 8 64-bit guest machine. To work around this problem, upgrade the machine type to rhel6.4.0 or later.

The x86emu emulator included in the Red Hat Enterprise Linux 5 version of X.Org fails to handle 32-bit prefixes correctly for a number of x86 assembler instructions contained in the vgabios. Consequently, when running a Red Hat Enterprise Linux 5 guest on a Red Hat Enterprise Linux 7 host, the vesa driver does not function on AMD64 and Intel 64 guests, and the X server on the guest machine fails to start. To work around this problem, use the native drivers for cirrus VGA and qxl VGA instead. In order to do this, it might be necessary to edit the /etc/X11/xorg.conf file and change the Driver entry in the Device section from vesa to cirrus or qxl.

When running Red Hat Enterprise Linux 4.9 as a guest virtual machine on a Red Hat Enterprise Linux 7 host, the guest is unable to detect VGA drivers. As a consequence, the GUI does not work. To work around this problem, access the Red Hat Enterprise Linux 4.9 server over an SSH connection using a command line interface, or upgrade the server to Red Hat Enterprise Linux 6 or Red Hat Enterprise Linux 7.

The X.Org version included in Red Hat Enterprise Linux 5 does not support automatic configuration of evdev devices. Consequently, when running a Red Hat Enterprise Linux 5 guest on a Red Hat Enterprise Linux 7 host, input devices fail to work unless they are configured manually. To work around this problem, edit the xorg.conf configuration file on the guest machine according to the following example for the USB tablet emulated by QEMU:

The virsh utility does not support system file-consistent snapshots for Microsoft Windows XP and Windows 2003 virtual guest machines. As a consequence, running the snapshot-create-as --quiesce --disk-only command fails. To work around this problem, do not use the --quiesce option. Please note that the snapshots will not be consistent with the guest file system.

Microsoft Windows 8.1 and Windows 2012 R2 systems require some CPU features that are not present in all qemu-kvm CPU models. Consequently, Microsoft Windows 8.1 and Windows 2012 R2 do not boot if certain CPU models are used, namely Opteron_G1, Conroe, and kvm64. To work around this problem, use CPU models that include the features required by Microsoft Windows 8.1 and Windows 2012 R2.

The QEMU Enhanced Disk format (QED) for KVM guest virtual machines is not supported in Red Hat Enterprise Linux 7. Use the qcow2 image format instead.

The SeaBIOS application runs in real mode for compatibility with BIOS interfaces. This limits the amount of memory available. As a consequence, SeaBIOS is only able to handle a limited number of disks. Currently, the supported number of disks is: As a workaround for this problem, when attaching a large number of disks to your virtual machine, make sure that your system disk has a small pci slot number, so SeaBIOS sees it first when scanning the pci bus. It is also recommended to use the virtio-scsi device instead of virtio-blk as the per-disk memory overhead is smaller.

When installing Red Hat Enterprise Linux 7 from sources other than the network, the network devices are not specified by default in the interface configuration files. As a consequence, creating a bridge by using the iface-bridge command in the virsh utility fails with an error message. To work around the problem, add the DEVICE= lines in the /etc/sysconfig/network-scripts/ifcfg-* files.

The emulated Small Computer System Interface (SCSI) driver, sym53c895, is not supported in Red Hat Enterprise Linux 7. As a consequence, a Red Hat Enterprise Linux 7 guest running under the Xen virtual machine monitor is currently unable to find devices, volumes, and images attached to the Red Hat Enterprise Linux 7 guest through emulated SCSI drivers. To work around this problem, it is possible to use paravirtualized drivers, xen-blkfront, as they are supported in Red Hat Enterprise Linux 7 and bring significant performance improvements. To do this, edit the Xen domain configuration file, usually found in the /etc/xen/ directory, and specify the disk in question as, for example, file:/root/raw.img,xvdc,w instead of file:/root/raw.img,sdc,w.

When using the serial console through KVM, holding down an arrow key for an extended period of time to navigate in the GRUB 2 menu results in erratic behavior. To work around this problem, avoid the rapid input caused by holding down an arrow key for a longer time.