6.4. 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.

When upgrading the NetKVM driver through the Windows Device Manager, the old registry values are not removed. As a consequence, for example, non-existent parameters may be available.

When working with very large images (larger than 2TB) created with very small cluster sizes (for example, 512bytes), block I/O errors can occur due to timeouts in qemu. To prevent this problem from occurring, use the default cluster size of 64KiB or larger.

On Microsoft Windows Server 2012 containing large dynamic VHDX (Hyper-V virtual hard disk) files and using the ext3 file system, a call trace can appear, and, consequently, it is not possible to shut down the guest. To work around this problem, use the ext4 file system or set a logical block size of 1MB when creating a VHDX file. Note that this can only be done by using Microsoft PowerShell as the Hyper-V manager does not expose the –BlockSizeBytes option which has the default value of 32MB. To create a dynamix VHDX file with an approximate size of 2.5TB and 1MB block size run:

New-VHD –Path .\MyDisk.vhdx –SizeBytes 5120MB –BlockSizeBytes 1MB -Dynamic

The storage drivers do not support the virsh vol-resize command options --allocate and --shrink. Use of the --shrink option will result in the following error message:

error: invalid argument: storageVolumeResize: unsupported flags (0x4)

Use of the --allocate option will result in the following error message:

error: invalid argument: storageVolumeResize: unsupported flags (0x1)

Shrinking a volume's capacity is possible as long as the value provided on the command line is greater than the volume allocation value as seen with the virsh vol-info command. You can shrink an existing volume by name through the followind sequence of steps:

In order to allocate more space on the volume, follow a similar sequence, but adjust the allocation to a larger value than the existing volume.

It is not possible to downgrade a driver using the Search for the best driver in these locations option because the newer and installed driver will be selected as the "best" driver. If you want to force installation of a particular driver version, use the Don't search option and the Have Disk button to select the folder of the older driver. This method will allow you to install an older driver on a system that already has a driver installed.

There is a known issue with the Microsoft Hyper-V host. If a legacy network interface controller (NIC) is used on a multiple-CPU virtual machine, there is an interrupt problem in the emulated hardware when the IRQ balancing daemon is running. Call trace information is logged in the /var/log/messages file.

Under certain circumstances, virtual machines try to boot from an incorrect device after a network boot failure. For more information, please refer to this article on Customer Portal.

If numad is run on a system with a task that has very large resident memory (>= 50% total system memory), then the numad-initiated NUMA page migrations for that task can cause swapping. The swapping can then induce long latencies for the system. An example is running a 256GB Microsoft Windows KVM Virtual Machine on a 512GB host. The Windows guest will fault in all pages on boot in order to zero them. On a four node system, numad will detect that a 256GB task can fit in a subset of two or three nodes, and then attempt to migrate it to that subset. Swapping can then occur and lead to latencies. These latencies may then cause the Windows guest to hang, as timing requirements are no longer met. Therefore, on a system with only one or two very large Windows machines, it is recommended to disable numad.

Note that this problem is specific to Windows 2012 guests that use more memory than exists in a single node. Windows 2012 guests appear to allocate memory more gradually than other Windows guest types, which triggers the issue. Other varieties of Windows guests do not seem to experience this problem. You can work around this problem by:

When a Red Hat Enterprise Linux 6.4 guest updates the kernel and then the guest is turned off through Microsoft Hyper-V Manager, the guest fails to boot due to incomplete grub information. This is because the data is not synced properly to disk when the machine is turned off through Hyper-V Manager. To work around this problem, execute the sync command before turning the guest off.

Using the mouse scroll wheel does not work on Red Hat Enterprise Linux 6.4 guests that run under certain version of Microsoft Hyper-V Manager. However, the scroll wheel works as expected when the vncviewer utility is used.

Microsoft Windows Server 2012 guests using the e1000 driver can become unresponsive consuming 100% CPU during boot or reboot.

When a kernel panic is triggered on a Microsoft Hyper-V guest, the kdump utility does not capture the kernel error information; an error is only displayed on the command line. This is a host problem. Guest kdump works as expected on Microsoft Hyper-V 2012 R2 host.

AMD Opteron G1, G2 or G3 CPU models on qemu-kvm use the family and models values as follows: family=15 and model=6. If these values are larger than 20, the lahfm_lm CPU feature is ignored by Linux guests, even when the feature is enabled. To work around this problem, use a different CPU model, for example AMD Opteron G4.

KVM guests must not be allowed to update the host CPU microcode. KVM does not allow this, and instead always returns the same microcode revision or patch level value to the guest. If the guest tries to update the CPU microcode, it will fail and show an error message similar to:

CPU0: update failed (for patch_level=0x6000624)

To work around this, configure the guest to not install CPU microcode updates; for example, uninstall the microcode_ctl package Red Hat Enterprise Linux of Fedora guests.

Converting a physical server running either Red Hat Enterprise Linux 4 or Red Hat Enterprise Linux 5 which has its file system root on an MD device is not supported. Converting such a guest results in a guest which fails to boot. Note that conversion of a Red Hat Enterprise Linux 6 server which has its root on an MD device is supported.

When converting a physical host with a multipath storage, Virt-P2V presents all available paths for conversion. Only a single path must be selected. This must be a currently active path.

The balloon service on Windows 7 guests can only be started by the Administrator user.

libvirt uses transient iptables rules for managing NAT or bridging to virtual machine guests. Any external command that reloads the iptables state (such as running system-config-firewall) will overwrite the entries needed by libvirt. Consequently, after running any command or tool that changes the state of iptables, guests may lose access to the network. To work around this issue, use the service libvirt reload command to restore libvirt's additional iptables rules.

A Windows virtual machine must be restarted after the installation of the kernel Windows driver framework. If the virtual machine is not restarted, it may crash when a memory balloon operation is performed.

Installation of Windows 7 Ultimate x86 (32-bit) Service Pack 1 on a guest with more than 4GB of RAM and more than one CPU from a DVD medium can lead to the system being unresponsive and, consequently, to a crash during the final steps of the installation process. To work around this issue, use the Windows Update utility to install the Service Pack.

A dual function Intel 82576 Gigabit Ethernet Controller interface (codename: Kawela, PCI Vendor/Device ID: 8086:10c9) cannot have both physical functions (PF's) device-assigned to a Windows 2008 guest. Either physical function can be device assigned to a Windows 2008 guest (PCI function 0 or function 1), but not both.

The virt-v2v utility is able to convert guests running on an ESX server. However, if an ESX guest has a disk with a snapshot, the snapshot must be on the same datastore as the underlying disk storage. If the snapshot and the underlying storage are on different datastores, virt-v2v will report a 404 error while trying to retrieve the storage.

The VMware Tools application on Microsoft Windows is unable to disable itself when it detects that it is no longer running on a VMware platform. Consequently, converting a Microsoft Windows guest from VMware ESX, which has VMware Tools installed, will result in errors. These errors usually manifest as error messages on start-up, and a "Stop Error" (also known as a BSOD) when shutting down the guest. To work around this issue, uninstall VMware Tools on Microsoft Windows guests prior to conversion.

The libguestfs packages do not support remote access to disks over the network in Red Hat Enterprise Linux 6. Consequently, the virt-sysprep tool as well as other tools do not work with remote disks. Users who need to access disks remotely with tools such as virt-sysprep are advised to upgrade to Red Hat Enterprise Linux 7.