305-300 Full Virtualization Practice Test 7 — 15 Questions

Question 1

When configuring a VM in libvirt, you want to ensure the highest performance for disk I/O. Which driver type should you select for the disk device?

Accepted Answer

Virtio. Virtio is a paravirtualized driver framework designed specifically for KVM/QEMU guests, allowing the guest to communicate with the host through shared memory rings rather than emulating real hardware. For disk I/O, virtio-blk (or virtio-scsi) bypasses much of the emulation overhead of IDE/SATA/SCSI, delivering significantly higher throughput and lower CPU utilization. That makes 'virtio' the correct choice when the goal is maximum disk I/O performance in libvirt.

Answer

SCSI

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IDE

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SATA

Question 2

Which THREE of the following are necessary components for a successful KVM virtualization environment on Linux?

Accepted Answer

The kvm kernel module. Option A (the kvm kernel module) is correct because KVM is implemented as a Linux kernel module that exposes the /dev/kvm interface and provides the core virtualization capability, so without loading kvm (or kvm_intel/kvm_amd) no guest can execute. Option B (QEMU userspace tools) is correct because KVM only handles CPU and memory virtualization in the kernel; QEMU provides the userspace device emulation, BIOS/firmware, and the qemu-system-x86_64 process that actually creates and runs the VM. Option C (hardware support for Intel VT-x or AMD-V) is correct because KVM requires CPU virtualization extensions (vmx or svm) exposed in the CPU flags; without them KVM cannot run hardware-accelerated guests. Option D (a dedicated storage network switch) is not required, since KVM guests can use local disks, files, or any existing network via virtio, and a separate storage switch is only an optional design choice. Option E (a full Xen hypervisor installed) is not required and is in fact an alternative hypervisor; KVM does not depend on Xen, and installing Xen would be a different virtualization stack.

Answer

A dedicated storage network switch

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A full Xen hypervisor installed

Question 3

You are migrating a running KVM guest from hostA to hostB with virsh migrate --live. The migration fails immediately with 'error: internal error: Attempt to migrate guest to the same host'. Both hosts share the same NFS storage and have identical CPU models. Which condition most likely explains this failure?

Accepted Answer

hostB resolves to the same IP address as hostA, or the migration URI points back to hostA.. libvirt compares the destination against the source host identity and aborts when they match, which is exactly what the 'same host' error indicates. The common causes are incorrect DNS or /etc/hosts entries, using a URI that resolves locally, or accidentally passing the local hostname as the destination. Confirming that hostB resolves to a distinct, reachable address and that the migration URI targets it resolves the issue.

Answer

The guest uses a virtio-net device without a configured migration address.

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The guest's disk image is stored on local storage instead of the shared NFS mount.

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The libvirt daemon on hostB is listening only on a Unix socket and not on TCP.

Question 4

You are configuring a KVM guest to use a paravirtualized clock and to report its IP addresses to the host so that libvirt can display them with virsh domifaddr. (Choose two.)

Accepted Answer

Install and run the qemu-guest-agent inside the guest and add the agent channel to the domain XML.. The paravirtualized clock is enabled through the kvmclock timer in the domain's clock configuration, and IP address reporting requires the QEMU guest agent running in the guest plus the associated virtio-serial channel. Together these two settings satisfy both requirements. ACPI, hostdev passthrough, and disk driver tuning address different concerns and do not provide either capability.

Answer

Set to enable ACPI shutdown handling in the guest.

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Add <driver name='qemu' type='raw'/> to the disk definition to improve guest timekeeping.

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Configure a <hostdev> entry for the guest's physical network interface to expose the IP address.

Question 5

A junior administrator needs to start the libvirt-managed guest 'db01' and confirm that it is running. Which pair of commands accomplishes this?

Accepted Answer

virsh start db01 and virsh list. Starting a persistent libvirt domain is done with virsh start, and the running state is confirmed with virsh list, which shows only active domains. The alternatives either use commands that expect XML files, list inactive domains as well, or never actually boot the guest, so they do not fulfill both parts of the task.

Answer

virsh define db01 and virsh dominfo db01

Answer

virsh start db01 and virsh list --all

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virsh create db01 and virsh list

Question 6

You are configuring a KVM host that will run several guests. You want each guest to be able to communicate with other guests and the host on the same subnet, while still allowing external hosts to reach the guests directly. You decide to use a libvirt virtual network with a bridge. Which libvirt network configuration element must be present to enable direct Layer 2 connectivity between the guests and the physical LAN?

Accepted Answer

<forward mode='bridge'/> with a bridge name that matches an existing host bridge.. A libvirt network with forward mode bridge and a bridge name that matches an existing host bridge places guest interfaces directly on the physical LAN bridge, giving them Layer 2 connectivity to other guests, the host, and external hosts. NAT, routed, and private modes either hide guests behind NAT, require Layer 3 routing, or isolate them entirely, so they do not provide the required direct connectivity.

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<forward mode='private'/> to isolate guest traffic from the host and external network.

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<forward mode='nat'/> with a private subnet and DHCP range.

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<forward mode='route'/> with a dedicated subnet and static routes on the physical router.

Question 7

A KVM host runs a Windows guest whose virtual disk is backed by a raw image file on an NFS mount. After a host reboot, the guest fails to start with 'Permission denied' on the disk path, although the file exists and is readable by root. The host uses SELinux enforcing mode. Which action resolves the problem?

Accepted Answer

Set the SELinux boolean 'virt_use_nfs' with 'setsebool -P virt_use_nfs on'.. A confined QEMU process cannot touch NFS storage unless the virt_use_nfs boolean is enabled, which is exactly what the Permission denied error after reboot indicates. The persistent form of setsebool applies the change across reboots. Relabeling local images, changing ownership, or disabling the security driver either targets the wrong path or weakens host security without solving the specific access denial.

Answer

Add 'security_driver = "none"' to /etc/libvirt/qemu.conf and restart libvirtd.

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Change the disk image ownership to 'qemu:qemu' with 'chown qemu:qemu' on the NFS-mounted file.

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Run 'restorecon -R -v /var/lib/libvirt/images' to reset file contexts on the local image directory.

Question 8

You are managing a KVM host and need to gracefully shut down a running guest named 'dbserver'. Which virsh command should you use?

Accepted Answer

virsh shutdown dbserver. The virsh shutdown command sends an ACPI shutdown request to the guest, allowing the guest operating system to perform a clean shutdown. This is the preferred method for gracefully stopping a virtual machine. The virsh destroy command forcibly powers off the guest, which can cause data loss. The other options are not valid virsh commands.

Answer

virsh stop dbserver

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virsh destroy dbserver

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virsh halt dbserver

Question 9

You administer a KVM host running libvirt. A developer needs a guest to be able to communicate with the host directly over a private network without any NAT or bridging to the physical LAN. You decide to use libvirt's default virtual network. Which virsh command will display the XML definition of this default network so you can inspect its forwarding mode and DHCP range?

Accepted Answer

virsh net-dumpxml default. The net-dumpxml subcommand is the canonical way to retrieve the XML configuration of a libvirt virtual network. Inspecting the default network's XML reveals its NAT forwarding mode and DHCP range, which is precisely what the scenario requires. Other net-* subcommands either list runtime state or leases rather than the full definition.

Answer

virsh net-dhcp-leases default

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virsh net-list --info default

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virsh iface-dumpxml default

Question 10

You are managing a KVM host and need to create a new storage pool that uses a directory on the host filesystem to store disk images. After creating the directory /var/lib/libvirt/images/extra, you want to define and start a libvirt storage pool named 'extra-pool' that uses this directory. Which sequence of virsh commands accomplishes this?

Accepted Answer

virsh pool-define-as extra-pool dir --target /var/lib/libvirt/images/extra; virsh pool-start extra-pool. The correct approach is to define a persistent directory-based pool using pool-define-as with type dir and the target path, then start it with pool-start. This creates a reusable pool definition and activates it immediately. Using pool-create-as would only create a transient pool, and the other commands either target volumes or use the wrong pool type.

Answer

virsh pool-create-as extra-pool dir --target /var/lib/libvirt/images/extra

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virsh vol-create-as extra-pool /var/lib/libvirt/images/extra 10G

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virsh pool-define-as extra-pool logical --target /var/lib/libvirt/images/extra

Question 11

You need to create a new storage pool for KVM guests using the virsh command-line tool. The pool should use a directory on the host filesystem at /var/lib/libvirt/images/extra. Which command correctly defines and starts this pool?

Accepted Answer

virsh pool-define-as extra dir --target /var/lib/libvirt/images/extra && virsh pool-start extra. To create a persistent directory-based storage pool, you use virsh pool-define-as with the pool name, type 'dir', and the target path. Then you start it with virsh pool-start. This ensures the pool is defined in libvirt's configuration and is immediately available for use. The pool-create-as command creates a transient pool that is lost on reboot, which is not suitable for a permanent addition.

Answer

virsh pool-create-as extra dir --target /var/lib/libvirt/images/extra

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virsh pool-define extra --type dir --path /var/lib/libvirt/images/extra && virsh pool-start extra

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virsh pool-add extra dir /var/lib/libvirt/images/extra && virsh pool-start extra

Question 12

You are troubleshooting a KVM guest that fails to boot after you changed its XML configuration. You want to verify and correct the guest's definition without starting it. Which TWO of the following actions are appropriate? (Choose two.)

Accepted Answer

Use virsh define /path/to/guest.xml to update the persistent domain definition from an edited XML file.. To correct a guest's persistent XML without starting it, you can either edit the XML directly with virsh edit, which validates before saving, or prepare an edited XML file and apply it with virsh define. Both update the persistent definition. Using virsh create would only start a transient guest, and undefining is destructive and unnecessary.

Answer

Use virsh undefine guestname then virsh define guest.xml to replace the configuration.

Answer

Use virsh dumpxml guestname > guest.xml, edit the file, then run virsh create guest.xml to apply changes persistently.

Answer

Use virsh start guestname --validate to check the XML and start the guest if valid.

Question 13

You are troubleshooting a KVM guest that fails to start with the error 'unsupported configuration: CPU mode 'host-passthrough' requires KVM'. Which of the following is the most likely cause?

Accepted Answer

The guest is configured to use a CPU mode that is only supported by the KVM hypervisor, but the hypervisor is not KVM (e.g., QEMU TCG).. The error 'unsupported configuration: CPU mode 'host-passthrough' requires KVM' occurs when the guest is configured with <cpu mode='host-passthrough'/> but libvirt is not using the KVM hypervisor. This can happen if KVM is not enabled on the host, or if the domain XML explicitly sets a different hypervisor type. host-passthrough exposes the host CPU model and features directly to the guest, which is only possible with hardware virtualization.

Answer

The host CPU is too old and does not support the host-passthrough mode.

Answer

The guest's XML configuration has a typo in the CPU mode attribute.

Answer

The host CPU does not support hardware virtualization extensions.

Question 14

A KVM guest is configured with a virtio-blk disk and a virtio-net device. Under heavy network load the guest CPU shows high system time inside the guest, and the host shows high context-switch rates for the QEMU process. The administrator wants to reduce per-packet I/O overhead without changing the guest OS drivers. Which change is most appropriate?

Accepted Answer

Enable vhost-net for the virtio-net device by ensuring the vhost_net kernel module is loaded.. Moving the virtio-net data plane into the kernel with vhost-net reduces the userspace context switches and system time that appear under heavy network load, while preserving the existing virtio-net guest driver. Changing the disk controller, emulating a slower NIC, or adding vCPUs does not target the packet-processing overhead and can even make host CPU usage worse.

Answer

Increase the number of vCPUs assigned to the guest to absorb the extra system time.

Answer

Change the network device model to e1000 to lower CPU usage on the host.

Answer

Switch the disk from virtio-blk to virtio-scsi and reattach the same image.

Question 15

You are setting up a KVM guest that requires very low latency for a real-time workload. You want to dedicate a physical CPU core to the guest and prevent the host from scheduling other tasks on that core. Which libvirt domain XML configuration achieves this?

Accepted Answer

combined with host kernel parameters isolcpus=2 and nohz_full=2.. To dedicate and isolate a physical core for a guest, you must both pin the vCPU to that core using vcpupin and configure the host kernel to exclude that core from general scheduling with isolcpus and nohz_full. Pinning alone only restricts where the guest runs; it does not prevent the host from using the core for other tasks. The combination provides the required isolation.

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