NAME
----
-qm - Qemu/KVM Virtual Machine Manager
+qm - QEMU/KVM Virtual Machine Manager
SYNOPSIS
-----------
endif::manvolnum[]
ifndef::manvolnum[]
-Qemu/KVM Virtual Machines
+QEMU/KVM Virtual Machines
=========================
:pve-toplevel:
endif::manvolnum[]
// http://pve.proxmox.com/wiki/KVM
// http://pve.proxmox.com/wiki/Qemu_Server
-Qemu (short form for Quick Emulator) is an open source hypervisor that emulates a
-physical computer. From the perspective of the host system where Qemu is
-running, Qemu is a user program which has access to a number of local resources
+QEMU (short form for Quick Emulator) is an open source hypervisor that emulates a
+physical computer. From the perspective of the host system where QEMU is
+running, QEMU is a user program which has access to a number of local resources
like partitions, files, network cards which are then passed to an
emulated computer which sees them as if they were real devices.
A guest operating system running in the emulated computer accesses these
devices, and runs as if it were running on real hardware. For instance, you can pass
-an ISO image as a parameter to Qemu, and the OS running in the emulated computer
+an ISO image as a parameter to QEMU, and the OS running in the emulated computer
will see a real CD-ROM inserted into a CD drive.
-Qemu can emulate a great variety of hardware from ARM to Sparc, but {pve} is
+QEMU can emulate a great variety of hardware from ARM to Sparc, but {pve} is
only concerned with 32 and 64 bits PC clone emulation, since it represents the
overwhelming majority of server hardware. The emulation of PC clones is also one
of the fastest due to the availability of processor extensions which greatly
-speed up Qemu when the emulated architecture is the same as the host
+speed up QEMU when the emulated architecture is the same as the host
architecture.
NOTE: You may sometimes encounter the term _KVM_ (Kernel-based Virtual Machine).
-It means that Qemu is running with the support of the virtualization processor
-extensions, via the Linux KVM module. In the context of {pve} _Qemu_ and
-_KVM_ can be used interchangeably, as Qemu in {pve} will always try to load the KVM
+It means that QEMU is running with the support of the virtualization processor
+extensions, via the Linux KVM module. In the context of {pve} _QEMU_ and
+_KVM_ can be used interchangeably, as QEMU in {pve} will always try to load the KVM
module.
-Qemu inside {pve} runs as a root process, since this is required to access block
+QEMU inside {pve} runs as a root process, since this is required to access block
and PCI devices.
Emulated devices and paravirtualized devices
--------------------------------------------
-The PC hardware emulated by Qemu includes a mainboard, network controllers,
+The PC hardware emulated by QEMU includes a mainboard, network controllers,
SCSI, IDE and SATA controllers, serial ports (the complete list can be seen in
the `kvm(1)` man page) all of them emulated in software. All these devices
are the exact software equivalent of existing hardware devices, and if the OS
running in the guest has the proper drivers it will use the devices as if it
-were running on real hardware. This allows Qemu to runs _unmodified_ operating
+were running on real hardware. This allows QEMU to runs _unmodified_ operating
systems.
This however has a performance cost, as running in software what was meant to
run in hardware involves a lot of extra work for the host CPU. To mitigate this,
-Qemu can present to the guest operating system _paravirtualized devices_, where
-the guest OS recognizes it is running inside Qemu and cooperates with the
+QEMU can present to the guest operating system _paravirtualized devices_, where
+the guest OS recognizes it is running inside QEMU and cooperates with the
hypervisor.
-Qemu relies on the virtio virtualization standard, and is thus able to present
+QEMU relies on the virtio virtualization standard, and is thus able to present
paravirtualized virtio devices, which includes a paravirtualized generic disk
controller, a paravirtualized network card, a paravirtualized serial port,
a paravirtualized SCSI controller, etc ...
-It is highly recommended to use the virtio devices whenever you can, as they
-provide a big performance improvement. Using the virtio generic disk controller
-versus an emulated IDE controller will double the sequential write throughput,
-as measured with `bonnie++(8)`. Using the virtio network interface can deliver
-up to three times the throughput of an emulated Intel E1000 network card, as
-measured with `iperf(1)`. footnote:[See this benchmark on the KVM wiki
-https://www.linux-kvm.org/page/Using_VirtIO_NIC]
+TIP: It is *highly recommended* to use the virtio devices whenever you can, as
+they provide a big performance improvement and are generally better maintained.
+Using the virtio generic disk controller versus an emulated IDE controller will
+double the sequential write throughput, as measured with `bonnie++(8)`. Using
+the virtio network interface can deliver up to three times the throughput of an
+emulated Intel E1000 network card, as measured with `iperf(1)`. footnote:[See
+this benchmark on the KVM wiki https://www.linux-kvm.org/page/Using_VirtIO_NIC]
[[qm_virtual_machines_settings]]
[thumbnail="screenshot/gui-create-vm-system.png"]
Additionally, the xref:qm_hard_disk[SCSI controller] can be changed.
If you plan to install the QEMU Guest Agent, or if your selected ISO image
-already ships and installs it automatically, you may want to tick the 'Qemu
+already ships and installs it automatically, you may want to tick the 'QEMU
Agent' box, which lets {pve} know that it can use its features to show some
more information, and complete some actions (for example, shutdown or
snapshots) more intelligently.
[[qm_hard_disk_bus]]
Bus/Controller
^^^^^^^^^^^^^^
-Qemu can emulate a number of storage controllers:
+QEMU can emulate a number of storage controllers:
+
+TIP: It is highly recommended to use the *VirtIO SCSI* or *VirtIO Block*
+controller for performance reasons and because they are better maintained.
* the *IDE* controller, has a design which goes back to the 1984 PC/AT disk
controller. Even if this controller has been superseded by recent designs,
hardware, and can connect up to 14 storage devices. {pve} emulates by default a
LSI 53C895A controller.
+
-A SCSI controller of type _VirtIO SCSI_ is the recommended setting if you aim for
-performance and is automatically selected for newly created Linux VMs since
-{pve} 4.3. Linux distributions have support for this controller since 2012, and
-FreeBSD since 2014. For Windows OSes, you need to provide an extra iso
-containing the drivers during the installation.
+A SCSI controller of type _VirtIO SCSI single_ and enabling the
+xref:qm_hard_disk_iothread[IO Thread] setting for the attached disks is
+recommended if you aim for performance. This is the default for newly created
+Linux VMs since {pve} 7.3. Each disk will have its own _VirtIO SCSI_ controller,
+and QEMU will handle the disks IO in a dedicated thread. Linux distributions
+have support for this controller since 2012, and FreeBSD since 2014. For Windows
+OSes, you need to provide an extra ISO containing the drivers during the
+installation.
// https://pve.proxmox.com/wiki/Paravirtualized_Block_Drivers_for_Windows#During_windows_installation.
-If you aim at maximum performance, you can select a SCSI controller of type
-_VirtIO SCSI single_ which will allow you to select the *IO Thread* option.
-When selecting _VirtIO SCSI single_ Qemu will create a new controller for
-each disk, instead of adding all disks to the same controller.
* The *VirtIO Block* controller, often just called VirtIO or virtio-blk,
is an older type of paravirtualized controller. It has been superseded by the
[[qm_hard_disk_iothread]]
IO Thread
^^^^^^^^^
-The option *IO Thread* can only be used when using a disk with the
-*VirtIO* controller, or with the *SCSI* controller, when the emulated controller
- type is *VirtIO SCSI single*.
-With this enabled, Qemu creates one I/O thread per storage controller,
-rather than a single thread for all I/O. This can increase performance when
-multiple disks are used and each disk has its own storage controller.
-
+The option *IO Thread* can only be used when using a disk with the *VirtIO*
+controller, or with the *SCSI* controller, when the emulated controller type is
+*VirtIO SCSI single*. With *IO Thread* enabled, QEMU creates one I/O thread per
+storage controller rather than handling all I/O in the main event loop or vCPU
+threads. One benefit is better work distribution and utilization of the
+underlying storage. Another benefit is reduced latency (hangs) in the guest for
+very I/O-intensive host workloads, since neither the main thread nor a vCPU
+thread can be blocked by disk I/O.
[[qm_cpu]]
CPU
Increasing the number of virtual CPUs (cores and sockets) will usually provide a
performance improvement though that is heavily dependent on the use of the VM.
Multi-threaded applications will of course benefit from a large number of
-virtual CPUs, as for each virtual cpu you add, Qemu will create a new thread of
+virtual CPUs, as for each virtual cpu you add, QEMU will create a new thread of
execution on the host system. If you're not sure about the workload of your VM,
it is usually a safe bet to set the number of *Total cores* to 2.
time in percent, so `1.0` is equal to `100%`, `2.5` to `250%` and so on. If a
single process would fully use one single core it would have `100%` CPU Time
usage. If a VM with four cores utilizes all its cores fully it would
-theoretically use `400%`. In reality the usage may be even a bit higher as Qemu
+theoretically use `400%`. In reality the usage may be even a bit higher as QEMU
can have additional threads for VM peripherals besides the vCPU core ones.
This setting can be useful if a VM should have multiple vCPUs, as it runs a few
processes in parallel, but the VM as a whole should not be able to run all
CPU Type
^^^^^^^^
-Qemu can emulate a number different of *CPU types* from 486 to the latest Xeon
+QEMU can emulate a number different of *CPU types* from 486 to the latest Xeon
processors. Each new processor generation adds new features, like hardware
assisted 3d rendering, random number generation, memory protection, etc ...
Usually you should select for your VM a processor type which closely matches the
This has a downside though. If you want to do a live migration of VMs between
different hosts, your VM might end up on a new system with a different CPU type.
If the CPU flags passed to the guest are missing, the qemu process will stop. To
-remedy this Qemu has also its own CPU type *kvm64*, that {pve} uses by defaults.
+remedy this QEMU has also its own CPU type *kvm64*, that {pve} uses by defaults.
kvm64 is a Pentium 4 look a like CPU type, which has a reduced CPU flags set,
but is guaranteed to work everywhere.
tap device is added to a bridge, by default vmbr0 in {pve}. In this mode, VMs
have direct access to the Ethernet LAN on which the host is located.
* in the alternative *NAT mode*, each virtual NIC will only communicate with
-the Qemu user networking stack, where a built-in router and DHCP server can
+the QEMU user networking stack, where a built-in router and DHCP server can
provide network access. This built-in DHCP will serve addresses in the private
10.0.2.0/24 range. The NAT mode is much slower than the bridged mode, and
should only be used for testing. This mode is only available via CLI or the API,
network device*.
You can overwrite the *MTU* setting for each VM network device. The option
-*mtu=1* represents a special case, in which the MTU value will be inherited
+`mtu=1` represents a special case, in which the MTU value will be inherited
from the underlying bridge.
This option is only available for *VirtIO* network devices.
standard setups.
Some operating systems (such as Windows 11) may require use of an UEFI
-compatible implementation instead. In such cases, you must rather use *OVMF*,
+compatible implementation. In such cases, you must use *OVMF* instead,
which is an open-source UEFI implementation. footnote:[See the OVMF Project https://github.com/tianocore/tianocore.github.io/wiki/OVMF]
There are other scenarios in which the SeaBIOS may not be the ideal firmware to
[[qm_qemu_agent]]
-Qemu Guest Agent
+QEMU Guest Agent
~~~~~~~~~~~~~~~~
-The Qemu Guest Agent is a service which runs inside the VM, providing a
+The QEMU Guest Agent is a service which runs inside the VM, providing a
communication channel between the host and the guest. It is used to exchange
information and allows the host to issue commands to the guest.
On a thin provisioned storage, this can help to free up unused space.
+NOTE: There is a caveat with ext4 on Linux, because it uses an in-memory
+optimization to avoid issuing duplicate TRIM requests. Since the guest doesn't
+know about the change in the underlying storage, only the first guest-trim will
+run as expected. Subsequent ones, until the next reboot, will only consider
+parts of the filesystem that changed since then.
+
Troubleshooting
^^^^^^^^^^^^^^^
Managing Virtual Machines with `qm`
------------------------------------
-qm is the tool to manage Qemu/Kvm virtual machines on {pve}. You can
+qm is the tool to manage QEMU/KVM virtual machines on {pve}. You can
create and destroy virtual machines, and control execution
(start/stop/suspend/resume). Besides that, you can use qm to set
parameters in the associated config file. It is also possible to
projects / pve-docs.git / blobdiff