storage: make description column wider

[pve-docs.git] / qm.adoc

[[chapter_virtual_machines]]
ifdef::manvolnum[]
qm(1)
=====
:pve-toplevel:

NAME
----

qm - Qemu/KVM Virtual Machine Manager


SYNOPSIS
--------

include::qm.1-synopsis.adoc[]

DESCRIPTION
-----------
endif::manvolnum[]
ifndef::manvolnum[]
Qemu/KVM Virtual Machines
=========================
:pve-toplevel:
endif::manvolnum[]

// deprecates
// http://pve.proxmox.com/wiki/Container_and_Full_Virtualization
// 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
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 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
will see a real CDROM inserted in a CD drive.

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
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
module.

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,
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
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
hypervisor.

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
http://www.linux-kvm.org/page/Using_VirtIO_NIC]


[[qm_virtual_machines_settings]]
Virtual Machines Settings
-------------------------

Generally speaking {pve} tries to choose sane defaults for virtual machines
(VM). Make sure you understand the meaning of the settings you change, as it
could incur a performance slowdown, or putting your data at risk.


[[qm_general_settings]]
General Settings
~~~~~~~~~~~~~~~~

[thumbnail="screenshot/gui-create-vm-general.png"]

General settings of a VM include

* the *Node* : the physical server on which the VM will run
* the *VM ID*: a unique number in this {pve} installation used to identify your VM
* *Name*: a free form text string you can use to describe the VM
* *Resource Pool*: a logical group of VMs


[[qm_os_settings]]
OS Settings
~~~~~~~~~~~

[thumbnail="screenshot/gui-create-vm-os.png"]

When creating a virtual machine (VM), setting the proper Operating System(OS)
allows {pve} to optimize some low level parameters. For instance Windows OS
expect the BIOS clock to use the local time, while Unix based OS expect the
BIOS clock to have the UTC time.

[[qm_system_settings]]
System Settings
~~~~~~~~~~~~~~~

On VM creation you can change some basic system components of the new VM. You
can specify which xref:qm_display[display type] you want to use.
[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
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.

{pve} allows to boot VMs with different firmware and machine types, namely
xref:qm_bios_and_uefi[SeaBIOS and OVMF]. In most cases you want to switch from
the default SeabBIOS to OVMF only if you plan to use
xref:qm_pci_passthrough[PCIe pass through]. A VMs 'Machine Type' defines the
hardware layout of the VM's virtual motherboard. You can choose between the
default https://en.wikipedia.org/wiki/Intel_440FX[Intel 440FX] or the
https://ark.intel.com/content/www/us/en/ark/products/31918/intel-82q35-graphics-and-memory-controller.html[Q35]
chipset, which also provides a virtual PCIe bus, and thus may be desired if
one wants to pass through PCIe hardware.

[[qm_hard_disk]]
Hard Disk
~~~~~~~~~

[[qm_hard_disk_bus]]
Bus/Controller
^^^^^^^^^^^^^^
Qemu can emulate a number of storage controllers:

* 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,
each and every OS you can think of has support for it, making it a great choice
if you want to run an OS released before 2003. You can connect up to 4 devices
on this controller.

* the *SATA* (Serial ATA) controller, dating from 2003, has a more modern
design, allowing higher throughput and a greater number of devices to be
connected. You can connect up to 6 devices on this controller.

* the *SCSI* controller, designed in 1985, is commonly found on server grade
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.
// 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
VirtIO SCSI Controller, in terms of features.

[thumbnail="screenshot/gui-create-vm-hard-disk.png"]

[[qm_hard_disk_formats]]
Image Format
^^^^^^^^^^^^
On each controller you attach a number of emulated hard disks, which are backed
by a file or a block device residing in the configured storage. The choice of
a storage type will determine the format of the hard disk image. Storages which
present block devices (LVM, ZFS, Ceph) will require the *raw disk image format*,
whereas files based storages (Ext4, NFS, CIFS, GlusterFS) will let you to choose
either the *raw disk image format* or the *QEMU image format*.

 * the *QEMU image format* is a copy on write format which allows snapshots, and
  thin provisioning of the disk image.
 * the *raw disk image* is a bit-to-bit image of a hard disk, similar to what
 you would get when executing the `dd` command on a block device in Linux. This
 format does not support thin provisioning or snapshots by itself, requiring
 cooperation from the storage layer for these tasks. It may, however, be up to
 10% faster than the *QEMU image format*. footnote:[See this benchmark for details
 http://events.linuxfoundation.org/sites/events/files/slides/CloudOpen2013_Khoa_Huynh_v3.pdf]
 * the *VMware image format* only makes sense if you intend to import/export the
 disk image to other hypervisors.

[[qm_hard_disk_cache]]
Cache Mode
^^^^^^^^^^
Setting the *Cache* mode of the hard drive will impact how the host system will
notify the guest systems of block write completions. The *No cache* default
means that the guest system will be notified that a write is complete when each
block reaches the physical storage write queue, ignoring the host page cache.
This provides a good balance between safety and speed.

If you want the {pve} backup manager to skip a disk when doing a backup of a VM,
you can set the *No backup* option on that disk.

If you want the {pve} storage replication mechanism to skip a disk when starting
 a replication job, you can set the *Skip replication* option on that disk.
As of {pve} 5.0, replication requires the disk images to be on a storage of type
`zfspool`, so adding a disk image to other storages when the VM has replication
configured requires to skip replication for this disk image.

[[qm_hard_disk_discard]]
Trim/Discard
^^^^^^^^^^^^
If your storage supports _thin provisioning_ (see the storage chapter in the
{pve} guide), you can activate the *Discard* option on a drive. With *Discard*
set and a _TRIM_-enabled guest OS footnote:[TRIM, UNMAP, and discard
https://en.wikipedia.org/wiki/Trim_%28computing%29], when the VM's filesystem
marks blocks as unused after deleting files, the controller will relay this
information to the storage, which will then shrink the disk image accordingly.
For the guest to be able to issue _TRIM_ commands, you must enable the *Discard*
option on the drive. Some guest operating systems may also require the
*SSD Emulation* flag to be set. Note that *Discard* on *VirtIO Block* drives is
only supported on guests using Linux Kernel 5.0 or higher.

If you would like a drive to be presented to the guest as a solid-state drive
rather than a rotational hard disk, you can set the *SSD emulation* option on
that drive. There is no requirement that the underlying storage actually be
backed by SSDs; this feature can be used with physical media of any type.
Note that *SSD emulation* is not supported on *VirtIO Block* drives.


[[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,
instead of a single thread for all I/O, so it can increase performance when
multiple isks are used and each disk has its own storage controller.


[[qm_cpu]]
CPU
~~~

[thumbnail="screenshot/gui-create-vm-cpu.png"]

A *CPU socket* is a physical slot on a PC motherboard where you can plug a CPU.
This CPU can then contain one or many *cores*, which are independent
processing units. Whether you have a single CPU socket with 4 cores, or two CPU
sockets with two cores is mostly irrelevant from a performance point of view.
However some software licenses depend on the number of sockets a machine has,
in that case it makes sense to set the number of sockets to what the license
allows you.

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.
Multithreaded 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
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.

NOTE: It is perfectly safe if the _overall_ number of cores of all your VMs
is greater than the number of cores on the server (e.g., 4 VMs with each 4
cores on a machine with only 8 cores). In that case the host system will
balance the Qemu execution threads between your server cores, just like if you
were running a standard multithreaded application. However, {pve} will prevent
you from starting VMs with more virtual CPU cores than physically available, as
this will only bring the performance down due to the cost of context switches.

[[qm_cpu_resource_limits]]
Resource Limits
^^^^^^^^^^^^^^^

In addition to the number of virtual cores, you can configure how much resources
a VM can get in relation to the host CPU time and also in relation to other
VMs.
With the *cpulimit* (``Host CPU Time'') option you can limit how much CPU time
the whole VM can use on the host. It is a floating point value representing CPU
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
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
vCPUs at 100% at the same time. Using a specific example: lets say we have a VM
which would profit from having 8 vCPUs, but at no time all of those 8 cores
should run at full load - as this would make the server so overloaded that
other VMs and CTs would get to less CPU. So, we set the *cpulimit* limit to
`4.0` (=400%). If all cores do the same heavy work they would all get 50% of a
real host cores CPU time. But, if only 4 would do work they could still get
almost 100% of a real core each.

NOTE: VMs can, depending on their configuration, use additional threads e.g.,
for networking or IO operations but also live migration. Thus a VM can show up
to use more CPU time than just its virtual CPUs could use. To ensure that a VM
never uses more CPU time than virtual CPUs assigned set the *cpulimit* setting
to the same value as the total core count.

The second CPU resource limiting setting, *cpuunits* (nowadays often called CPU
shares or CPU weight), controls how much CPU time a VM gets in regards to other
VMs running.  It is a relative weight which defaults to `1024`, if you increase
this for a VM it will be prioritized by the scheduler in comparison to other
VMs with lower weight. E.g., if VM 100 has set the default 1024 and VM 200 was
changed to `2048`, the latter VM 200 would receive twice the CPU bandwidth than
the first VM 100.

For more information see `man systemd.resource-control`, here `CPUQuota`
corresponds to `cpulimit` and `CPUShares` corresponds to our `cpuunits`
setting, visit its Notes section for references and implementation details.

CPU Type
^^^^^^^^

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
CPU of the host system, as it means that the host CPU features (also called _CPU
flags_ ) will be available in your VMs. If you want an exact match, you can set
the CPU type to *host* in which case the VM will have exactly the same CPU flags
as your host system.

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.
kvm64 is a Pentium 4 look a like CPU type, which has a reduced CPU flags set,
but is guaranteed to work everywhere.

In short, if you care about live migration and moving VMs between nodes, leave
the kvm64 default. If you don’t care about live migration or have a homogeneous
cluster where all nodes have the same CPU, set the CPU type to host, as in
theory this will give your guests maximum performance.

Meltdown / Spectre related CPU flags
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

There are several CPU flags related to the Meltdown and Spectre vulnerabilities
footnote:[Meltdown Attack https://meltdownattack.com/] which need to be set
manually unless the selected CPU type of your VM already enables them by default.

There are two requirements that need to be fulfilled in order to use these
CPU flags:

* The host CPU(s) must support the feature and propagate it to the guest's virtual CPU(s)
* The guest operating system must be updated to a version which mitigates the
  attacks and is able to utilize the CPU feature

Otherwise you need to set the desired CPU flag of the virtual CPU, either by
editing the CPU options in the WebUI, or by setting the 'flags' property of the
'cpu' option in the VM configuration file.

For Spectre v1,v2,v4 fixes, your CPU or system vendor also needs to provide a
so-called ``microcode update'' footnote:[You can use `intel-microcode' /
`amd-microcode' from Debian non-free if your vendor does not provide such an
update. Note that not all affected CPUs can be updated to support spec-ctrl.]
for your CPU.


To check if the {pve} host is vulnerable, execute the following command as root:

----
for f in /sys/devices/system/cpu/vulnerabilities/*; do echo "${f##*/} -" $(cat "$f"); done
----

A community script is also available to detect is the host is still vulnerable.
footnote:[spectre-meltdown-checker https://meltdown.ovh/]

Intel processors
^^^^^^^^^^^^^^^^

* 'pcid'
+
This reduces the performance impact of the Meltdown (CVE-2017-5754) mitigation
called 'Kernel Page-Table Isolation (KPTI)', which effectively hides
the Kernel memory from the user space. Without PCID, KPTI is quite an expensive
mechanism footnote:[PCID is now a critical performance/security feature on x86
https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU].
+
To check if the {pve} host supports PCID, execute the following command as root:
+
----
# grep ' pcid ' /proc/cpuinfo
----
+
If this does not return empty your host's CPU has support for 'pcid'.

* 'spec-ctrl'
+
Required to enable the Spectre v1 (CVE-2017-5753) and Spectre v2 (CVE-2017-5715) fix,
in cases where retpolines are not sufficient.
Included by default in Intel CPU models with -IBRS suffix.
Must be explicitly turned on for Intel CPU models without -IBRS suffix.
Requires an updated host CPU microcode (intel-microcode >= 20180425).
+
* 'ssbd'
+
Required to enable the Spectre V4 (CVE-2018-3639) fix. Not included by default in any Intel CPU model.
Must be explicitly turned on for all Intel CPU models.
Requires an updated host CPU microcode(intel-microcode >= 20180703).


AMD processors
^^^^^^^^^^^^^^

* 'ibpb'
+
Required to enable the Spectre v1 (CVE-2017-5753) and Spectre v2 (CVE-2017-5715) fix,
in cases where retpolines are not sufficient.
Included by default in AMD CPU models with -IBPB suffix.
Must be explicitly turned on for AMD CPU models without -IBPB suffix.
Requires the host CPU microcode to support this feature before it can be used for guest CPUs.



* 'virt-ssbd'
+
Required to enable the Spectre v4 (CVE-2018-3639) fix.
Not included by default in any AMD CPU model.
Must be explicitly turned on for all AMD CPU models.
This should be provided to guests, even if amd-ssbd is also provided, for maximum guest compatibility.
Note that this must be explicitly enabled when when using the "host" cpu model,
because this is a virtual feature which does not exist in the physical CPUs.


* 'amd-ssbd'
+
Required to enable the Spectre v4 (CVE-2018-3639) fix.
Not included by default in any AMD CPU model. Must be explicitly turned on for all AMD CPU models.
This provides higher performance than virt-ssbd, therefore a host supporting this should always expose this to guests if possible.
virt-ssbd should none the less also be exposed for maximum guest compatibility as some kernels only know about virt-ssbd.


* 'amd-no-ssb'
+
Recommended to indicate the host is not vulnerable to Spectre V4 (CVE-2018-3639).
Not included by default in any AMD CPU model.
Future hardware generations of CPU will not be vulnerable to CVE-2018-3639,
and thus the guest should be told not to enable its mitigations, by exposing amd-no-ssb.
This is mutually exclusive with virt-ssbd and amd-ssbd.


NUMA
^^^^
You can also optionally emulate a *NUMA*
footnote:[https://en.wikipedia.org/wiki/Non-uniform_memory_access] architecture
in your VMs. The basics of the NUMA architecture mean that instead of having a
global memory pool available to all your cores, the memory is spread into local
banks close to each socket.
This can bring speed improvements as the memory bus is not a bottleneck
anymore. If your system has a NUMA architecture footnote:[if the command
`numactl --hardware | grep available` returns more than one node, then your host
system has a NUMA architecture] we recommend to activate the option, as this
will allow proper distribution of the VM resources on the host system.
This option is also required to hot-plug cores or RAM in a VM.

If the NUMA option is used, it is recommended to set the number of sockets to
the number of nodes of the host system.

vCPU hot-plug
^^^^^^^^^^^^^

Modern operating systems introduced the capability to hot-plug and, to a
certain extent, hot-unplug CPUs in a running systems. Virtualisation allows us
to avoid a lot of the (physical) problems real hardware can cause in such
scenarios.
Still, this is a rather new and complicated feature, so its use should be
restricted to cases where its absolutely needed. Most of the functionality can
be replicated with other, well tested and less complicated, features, see
xref:qm_cpu_resource_limits[Resource Limits].

In {pve} the maximal number of plugged CPUs is always `cores * sockets`.
To start a VM with less than this total core count of CPUs you may use the
*vpus* setting, it denotes how many vCPUs should be plugged in at VM start.

Currently only this feature is only supported on Linux, a kernel newer than 3.10
is needed, a kernel newer than 4.7 is recommended.

You can use a udev rule as follow to automatically set new CPUs as online in
the guest:

----
SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ATTR{online}="1"
----

Save this under /etc/udev/rules.d/ as a file ending in `.rules`.

Note: CPU hot-remove is machine dependent and requires guest cooperation.
The deletion command does not guarantee CPU removal to actually happen,
typically it's a request forwarded to guest using target dependent mechanism,
e.g., ACPI on x86/amd64.


[[qm_memory]]
Memory
~~~~~~

For each VM you have the option to set a fixed size memory or asking
{pve} to dynamically allocate memory based on the current RAM usage of the
host.

.Fixed Memory Allocation
[thumbnail="screenshot/gui-create-vm-memory.png"]

When setting memory and minimum memory to the same amount
{pve} will simply allocate what you specify to your VM.

Even when using a fixed memory size, the ballooning device gets added to the
VM, because it delivers useful information such as how much memory the guest
really uses.
In general, you should leave *ballooning* enabled, but if you want to disable
it (e.g. for debugging purposes), simply uncheck
*Ballooning Device* or set

 balloon: 0

in the configuration.

.Automatic Memory Allocation

// see autoballoon() in pvestatd.pm
When setting the minimum memory lower than memory, {pve} will make sure that the
minimum amount you specified is always available to the VM, and if RAM usage on
the host is below 80%, will dynamically add memory to the guest up to the
maximum memory specified.

When the host is running low on RAM, the VM will then release some memory
back to the host, swapping running processes if needed and starting the oom
killer in last resort. The passing around of memory between host and guest is
done via a special `balloon` kernel driver running inside the guest, which will
grab or release memory pages from the host.
footnote:[A good explanation of the inner workings of the balloon driver can be found here https://rwmj.wordpress.com/2010/07/17/virtio-balloon/]

When multiple VMs use the autoallocate facility, it is possible to set a
*Shares* coefficient which indicates the relative amount of the free host memory
that each VM should take. Suppose for instance you have four VMs, three of them
running an HTTP server and the last one is a database server. To cache more
database blocks in the database server RAM, you would like to prioritize the
database VM when spare RAM is available. For this you assign a Shares property
of 3000 to the database VM, leaving the other VMs to the Shares default setting
of 1000. The host server has 32GB of RAM, and is currently using 16GB, leaving 32
* 80/100 - 16 = 9GB RAM to be allocated to the VMs. The database VM will get 9 *
3000 / (3000 + 1000 + 1000 + 1000) = 4.5 GB extra RAM and each HTTP server will
get 1.5 GB.

All Linux distributions released after 2010 have the balloon kernel driver
included. For Windows OSes, the balloon driver needs to be added manually and can
incur a slowdown of the guest, so we don't recommend using it on critical
systems.
// see https://forum.proxmox.com/threads/solved-hyper-threading-vs-no-hyper-threading-fixed-vs-variable-memory.20265/

When allocating RAM to your VMs, a good rule of thumb is always to leave 1GB
of RAM available to the host.


[[qm_network_device]]
Network Device
~~~~~~~~~~~~~~

[thumbnail="screenshot/gui-create-vm-network.png"]

Each VM can have many _Network interface controllers_ (NIC), of four different
types:

 * *Intel E1000* is the default, and emulates an Intel Gigabit network card.
 * the *VirtIO* paravirtualized NIC should be used if you aim for maximum
performance. Like all VirtIO devices, the guest OS should have the proper driver
installed.
 * the *Realtek 8139* emulates an older 100 MB/s network card, and should
only be used when emulating older operating systems ( released before 2002 )
 * the *vmxnet3* is another paravirtualized device, which should only be used
when importing a VM from another hypervisor.

{pve} will generate for each NIC a random *MAC address*, so that your VM is
addressable on Ethernet networks.

The NIC you added to the VM can follow one of two different models:

 * in the default *Bridged mode* each virtual NIC is backed on the host by a
_tap device_, ( a software loopback device simulating an Ethernet NIC ). This
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
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,
but not via the WebUI.

You can also skip adding a network device when creating a VM by selecting *No
network device*.

.Multiqueue
If you are using the VirtIO driver, you can optionally activate the
*Multiqueue* option. This option allows the guest OS to process networking
packets using multiple virtual CPUs, providing an increase in the total number
of packets transferred.

//http://blog.vmsplice.net/2011/09/qemu-internals-vhost-architecture.html
When using the VirtIO driver with {pve}, each NIC network queue is passed to the
host kernel, where the queue will be processed by a kernel thread spawned by the
vhost driver. With this option activated, it is possible to pass _multiple_
network queues to the host kernel for each NIC.

//https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Virtualization_Tuning_and_Optimization_Guide/sect-Virtualization_Tuning_Optimization_Guide-Networking-Techniques.html#sect-Virtualization_Tuning_Optimization_Guide-Networking-Multi-queue_virtio-net
When using Multiqueue, it is recommended to set it to a value equal
to the number of Total Cores of your guest. You also need to set in
the VM the number of multi-purpose channels on each VirtIO NIC with the ethtool
command:

`ethtool -L ens1 combined X`

where X is the number of the number of vcpus of the VM.

You should note that setting the Multiqueue parameter to a value greater
than one will increase the CPU load on the host and guest systems as the
traffic increases. We recommend to set this option only when the VM has to
process a great number of incoming connections, such as when the VM is running
as a router, reverse proxy or a busy HTTP server doing long polling.

[[qm_display]]
Display
~~~~~~~

QEMU can virtualize a few types of VGA hardware. Some examples are:

* *std*, the default, emulates a card with Bochs VBE extensions.
* *cirrus*, this was once the default, it emulates a very old hardware module
with all its problems. This display type should only be used if really
necessary footnote:[https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/
qemu: using cirrus considered harmful], e.g., if using Windows XP or earlier
* *vmware*, is a VMWare SVGA-II compatible adapter.
* *qxl*, is the QXL paravirtualized graphics card. Selecting this also
enables https://www.spice-space.org/[SPICE] (a remote viewer protocol) for the
VM.

You can edit the amount of memory given to the virtual GPU, by setting
the 'memory' option. This can enable higher resolutions inside the VM,
especially with SPICE/QXL.

As the memory is reserved by display device, selecting Multi-Monitor mode
for SPICE (e.g., `qxl2` for dual monitors) has some implications:

* Windows needs a device for each monitor, so if your 'ostype' is some
version of Windows, {pve} gives the VM an extra device per monitor.
Each device gets the specified amount of memory.

* Linux VMs, can always enable more virtual monitors, but selecting
a Multi-Monitor mode multiplies the memory given to the device with
the number of monitors.

Selecting `serialX` as display 'type' disables the VGA output, and redirects
the Web Console to the selected serial port. A configured display 'memory'
setting will be ignored in that case.

[[qm_usb_passthrough]]
USB Passthrough
~~~~~~~~~~~~~~~

There are two different types of USB passthrough devices:

* Host USB passthrough
* SPICE USB passthrough

Host USB passthrough works by giving a VM a USB device of the host.
This can either be done via the vendor- and product-id, or
via the host bus and port.

The vendor/product-id looks like this: *0123:abcd*,
where *0123* is the id of the vendor, and *abcd* is the id
of the product, meaning two pieces of the same usb device
have the same id.

The bus/port looks like this: *1-2.3.4*, where *1* is the bus
and *2.3.4* is the port path. This represents the physical
ports of your host (depending of the internal order of the
usb controllers).

If a device is present in a VM configuration when the VM starts up,
but the device is not present in the host, the VM can boot without problems.
As soon as the device/port is available in the host, it gets passed through.

WARNING: Using this kind of USB passthrough means that you cannot move
a VM online to another host, since the hardware is only available
on the host the VM is currently residing.

The second type of passthrough is SPICE USB passthrough. This is useful
if you use a SPICE client which supports it. If you add a SPICE USB port
to your VM, you can passthrough a USB device from where your SPICE client is,
directly to the VM (for example an input device or hardware dongle).


[[qm_bios_and_uefi]]
BIOS and UEFI
~~~~~~~~~~~~~

In order to properly emulate a computer, QEMU needs to use a firmware.
Which, on common PCs often known as BIOS or (U)EFI, is executed as one of the
first steps when booting a VM. It is responsible for doing basic hardware
initialization and for providing an interface to the firmware and hardware for
the operating system. By default QEMU uses *SeaBIOS* for this, which is an
open-source, x86 BIOS implementation. SeaBIOS is a good choice for most
standard setups.

There are, however, some scenarios in which a BIOS is not a good firmware
to boot from, e.g. if you want to do VGA passthrough. footnote:[Alex Williamson has a very good blog entry about this.
http://vfio.blogspot.co.at/2014/08/primary-graphics-assignment-without-vga.html]
In such cases, you should rather use *OVMF*, which is an open-source UEFI implementation. footnote:[See the OVMF Project http://www.tianocore.org/ovmf/]

If you want to use OVMF, there are several things to consider:

In order to save things like the *boot order*, there needs to be an EFI Disk.
This disk will be included in backups and snapshots, and there can only be one.

You can create such a disk with the following command:

 qm set <vmid> -efidisk0 <storage>:1,format=<format>

Where *<storage>* is the storage where you want to have the disk, and
*<format>* is a format which the storage supports. Alternatively, you can
create such a disk through the web interface with 'Add' -> 'EFI Disk' in the
hardware section of a VM.

When using OVMF with a virtual display (without VGA passthrough),
you need to set the client resolution in the OVMF menu(which you can reach
with a press of the ESC button during boot), or you have to choose
SPICE as the display type.

[[qm_ivshmem]]
Inter-VM shared memory
~~~~~~~~~~~~~~~~~~~~~~

You can add an Inter-VM shared memory device (`ivshmem`), which allows one to
share memory between the host and a guest, or also between multiple guests.

To add such a device, you can use `qm`:

  qm set <vmid> -ivshmem size=32,name=foo

Where the size is in MiB. The file will be located under
`/dev/shm/pve-shm-$name` (the default name is the vmid).

NOTE: Currently the device will get deleted as soon as any VM using it got
shutdown or stopped. Open connections will still persist, but new connections
to the exact same device cannot be made anymore.

A use case for such a device is the Looking Glass
footnote:[Looking Glass: https://looking-glass.hostfission.com/] project,
which enables high performance, low-latency display mirroring between
host and guest.

[[qm_audio_device]]
Audio Device
~~~~~~~~~~~~

To add an audio device run the following command:

----
qm set <vmid> -audio0 device=<device>
----

Supported audio devices are:

* `ich9-intel-hda`: Intel HD Audio Controller, emulates ICH9
* `intel-hda`: Intel HD Audio Controller, emulates ICH6
* `AC97`: Audio Codec '97, useful for older operating systems like Windows XP

NOTE: The audio device works only in combination with SPICE. Remote protocols
like Microsoft's RDP have options to play sound. To use the physical audio
device of the host use device passthrough (see
xref:qm_pci_passthrough[PCI Passthrough] and
xref:qm_usb_passthrough[USB Passthrough]).

[[qm_virtio_rng]]
VirtIO RNG
~~~~~~~~~~

A RNG (Random Number Generator) is a device providing entropy ('randomness') to
a system. A virtual hardware-RNG can be used to provide such entropy from the
host system to a guest VM. This helps to avoid entropy starvation problems in
the guest (a situation where not enough entropy is available and the system may
slow down or run into problems), especially during the guests boot process.

To add a VirtIO-based emulated RNG, run the following command:

----
qm set <vmid> -rng0 source=<source>[,max_bytes=X,period=Y]
----

`source` specifies where entropy is read from on the host and has to be one of
the following:

* `/dev/urandom`: Non-blocking kernel entropy pool (preferred)
* `/dev/random`: Blocking kernel pool (not recommended, can lead to entropy
  starvation on the host system)
* `/dev/hwrng`: To pass through a hardware RNG attached to the host (if multiple
  are available, the one selected in
  `/sys/devices/virtual/misc/hw_random/rng_current` will be used)

A limit can be specified via the `max_bytes` and `period` parameters, they are
read as `max_bytes` per `period` in milliseconds. However, it does not represent
a linear relationship: 1024B/1000ms would mean that up to 1 KiB of data becomes
available on a 1 second timer, not that 1 KiB is streamed to the guest over the
course of one second. Reducing the `period` can thus be used to inject entropy
into the guest at a faster rate.

By default, the limit is set to 1024 bytes per 1000 ms (1 KiB/s). It is
recommended to always use a limiter to avoid guests using too many host
resources. If desired, a value of '0' for `max_bytes` can be used to disable
all limits.

[[qm_startup_and_shutdown]]
Automatic Start and Shutdown of Virtual Machines
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After creating your VMs, you probably want them to start automatically
when the host system boots. For this you need to select the option 'Start at
boot' from the 'Options' Tab of your VM in the web interface, or set it with
the following command:

 qm set <vmid> -onboot 1

.Start and Shutdown Order

[thumbnail="screenshot/gui-qemu-edit-start-order.png"]

In some case you want to be able to fine tune the boot order of your
VMs, for instance if one of your VM is providing firewalling or DHCP
to other guest systems.  For this you can use the following
parameters:

* *Start/Shutdown order*: Defines the start order priority. E.g. set it to 1 if
you want the VM to be the first to be started. (We use the reverse startup
order for shutdown, so a machine with a start order of 1 would be the last to
be shut down). If multiple VMs have the same order defined on a host, they will
additionally be ordered by 'VMID' in ascending order.
* *Startup delay*: Defines the interval between this VM start and subsequent
VMs starts . E.g. set it to 240 if you want to wait 240 seconds before starting
other VMs.
* *Shutdown timeout*: Defines the duration in seconds {pve} should wait
for the VM to be offline after issuing a shutdown command.
By default this value is set to 180, which means that {pve} will issue a
shutdown request and wait 180 seconds for the machine to be offline. If
the machine is still online after the timeout it will be stopped forcefully.

NOTE: VMs managed by the HA stack do not follow the 'start on boot' and
'boot order' options currently. Those VMs will be skipped by the startup and
shutdown algorithm as the HA manager itself ensures that VMs get started and
stopped.

Please note that machines without a Start/Shutdown order parameter will always
start after those where the parameter is set. Further, this parameter can only
be enforced between virtual machines running on the same host, not
cluster-wide.

[[qm_spice_enhancements]]
SPICE Enhancements
~~~~~~~~~~~~~~~~~~

SPICE Enhancements are optional features that can improve the remote viewer
experience.

To enable them via the GUI go to the *Options* panel of the virtual machine. Run
the following command to enable them via the CLI:

----
qm set <vmid> -spice_enhancements foldersharing=1,videostreaming=all
----

NOTE: To use these features the <<qm_display,*Display*>> of the virtual machine
must be set to SPICE (qxl).

Folder Sharing
^^^^^^^^^^^^^^

Share a local folder with the guest. The `spice-webdavd` daemon needs to be
installed in the guest. It makes the shared folder available through a local
WebDAV server located at http://localhost:9843.

For Windows guests the installer for the 'Spice WebDAV daemon' can be downloaded
from the
https://www.spice-space.org/download.html#windows-binaries[official SPICE website].

Most Linux distributions have a package called `spice-webdavd` that can be
installed.

To share a folder in Virt-Viewer (Remote Viewer) go to 'File -> Preferences'.
Select the folder to share and then enable the checkbox.

NOTE: Folder sharing currently only works in the Linux version of Virt-Viewer.

CAUTION: Experimental! Currently this feature does not work reliably.

Video Streaming
^^^^^^^^^^^^^^^

Fast refreshing areas are encoded into a video stream. Two options exist:

* *all*: Any fast refreshing area will be encoded into a video stream.
* *filter*: Additional filters are used to decide if video streaming should be
  used (currently only small window surfaces are skipped).

A general recommendation if video streaming should be enabled and which option
to choose from cannot be given. Your mileage may vary depending on the specific
circumstances.

Troubleshooting
^^^^^^^^^^^^^^^

.Shared folder does not show up

Make sure the WebDAV service is enabled and running in the guest. On Windows it
is called 'Spice webdav proxy'. In Linux the name is 'spice-webdavd' but can be
different depending on the distribution.

If the service is running, check the WebDAV server by opening
http://localhost:9843 in a browser in the guest.

It can help to restart the SPICE session.

[[qm_migration]]
Migration
---------

[thumbnail="screenshot/gui-qemu-migrate.png"]

If you have a cluster, you can migrate your VM to another host with

 qm migrate <vmid> <target>

There are generally two mechanisms for this

* Online Migration (aka Live Migration)
* Offline Migration

Online Migration
~~~~~~~~~~~~~~~~

When your VM is running and it has no local resources defined (such as disks
on local storage, passed through devices, etc.) you can initiate a live
migration with the -online flag.

How it works
^^^^^^^^^^^^

This starts a Qemu Process on the target host with the 'incoming' flag, which
means that the process starts and waits for the memory data and device states
from the source Virtual Machine (since all other resources, e.g. disks,
are shared, the memory content and device state are the only things left
to transmit).

Once this connection is established, the source begins to send the memory
content asynchronously to the target. If the memory on the source changes,
those sections are marked dirty and there will be another pass of sending data.
This happens until the amount of data to send is so small that it can
pause the VM on the source, send the remaining data to the target and start
the VM on the target in under a second.

Requirements
^^^^^^^^^^^^

For Live Migration to work, there are some things required:

* The VM has no local resources (e.g. passed through devices, local disks, etc.)
* The hosts are in the same {pve} cluster.
* The hosts have a working (and reliable) network connection.
* The target host must have the same or higher versions of the
  {pve} packages. (It *might* work the other way, but this is never guaranteed)

Offline Migration
~~~~~~~~~~~~~~~~~

If you have local resources, you can still offline migrate your VMs,
as long as all disk are on storages, which are defined on both hosts.
Then the migration will copy the disk over the network to the target host.

[[qm_copy_and_clone]]
Copies and Clones
-----------------

[thumbnail="screenshot/gui-qemu-full-clone.png"]

VM installation is usually done using an installation media (CD-ROM)
from the operation system vendor. Depending on the OS, this can be a
time consuming task one might want to avoid.

An easy way to deploy many VMs of the same type is to copy an existing
VM. We use the term 'clone' for such copies, and distinguish between
'linked' and 'full' clones.

Full Clone::

The result of such copy is an independent VM. The
new VM does not share any storage resources with the original.
+

It is possible to select a *Target Storage*, so one can use this to
migrate a VM to a totally different storage. You can also change the
disk image *Format* if the storage driver supports several formats.
+

NOTE: A full clone needs to read and copy all VM image data. This is
usually much slower than creating a linked clone.
+

Some storage types allows to copy a specific *Snapshot*, which
defaults to the 'current' VM data. This also means that the final copy
never includes any additional snapshots from the original VM.


Linked Clone::

Modern storage drivers support a way to generate fast linked
clones. Such a clone is a writable copy whose initial contents are the
same as the original data. Creating a linked clone is nearly
instantaneous, and initially consumes no additional space.
+

They are called 'linked' because the new image still refers to the
original. Unmodified data blocks are read from the original image, but
modification are written (and afterwards read) from a new
location. This technique is called 'Copy-on-write'.
+

This requires that the original volume is read-only. With {pve} one
can convert any VM into a read-only <<qm_templates, Template>>). Such
templates can later be used to create linked clones efficiently.
+

NOTE: You cannot delete an original template while linked clones
exist.
+

It is not possible to change the *Target storage* for linked clones,
because this is a storage internal feature.


The *Target node* option allows you to create the new VM on a
different node. The only restriction is that the VM is on shared
storage, and that storage is also available on the target node.

To avoid resource conflicts, all network interface MAC addresses get
randomized, and we generate a new 'UUID' for the VM BIOS (smbios1)
setting.


[[qm_templates]]
Virtual Machine Templates
-------------------------

One can convert a VM into a Template. Such templates are read-only,
and you can use them to create linked clones.

NOTE: It is not possible to start templates, because this would modify
the disk images. If you want to change the template, create a linked
clone and modify that.

VM Generation ID
----------------

{pve} supports Virtual Machine Generation ID ('vmgenid') footnote:[Official
'vmgenid' Specification
https://docs.microsoft.com/en-us/windows/desktop/hyperv_v2/virtual-machine-generation-identifier]
for virtual machines.
This can be used by the guest operating system to detect any event resulting
in a time shift event, for example, restoring a backup or a snapshot rollback.

When creating new VMs, a 'vmgenid' will be automatically generated and saved
in its configuration file.

To create and add a 'vmgenid' to an already existing VM one can pass the
special value `1' to let {pve} autogenerate one or manually set the 'UUID'
footnote:[Online GUID generator http://guid.one/] by using it as value,
e.g.:

----
 qm set VMID -vmgenid 1
 qm set VMID -vmgenid 00000000-0000-0000-0000-000000000000
----

NOTE: The initial addition of a 'vmgenid' device to an existing VM, may result
in the same effects as a change on snapshot rollback, backup restore, etc., has
as the VM can interpret this as generation change.

In the rare case the 'vmgenid' mechanism is not wanted one can pass `0' for
its value on VM creation, or retroactively delete the property in the
configuration with:

----
 qm set VMID -delete vmgenid
----

The most prominent use case for 'vmgenid' are newer Microsoft Windows
operating systems, which use it to avoid problems in time sensitive or
replicate services (e.g., databases, domain controller
footnote:[https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/get-started/virtual-dc/virtualized-domain-controller-architecture])
on snapshot rollback, backup restore or a whole VM clone operation.

Importing Virtual Machines and disk images
------------------------------------------

A VM export from a foreign hypervisor takes usually the form of one or more disk
 images, with a configuration file describing the settings of the VM (RAM,
 number of cores). +
The disk images can be in the vmdk format, if the disks come from
VMware or VirtualBox, or qcow2 if the disks come from a KVM hypervisor.
The most popular configuration format for VM exports is the OVF standard, but in
practice interoperation is limited because many settings are not implemented in
the standard itself, and hypervisors export the supplementary information
in non-standard extensions.

Besides the problem of format, importing disk images from other hypervisors
may fail if the emulated hardware changes too much from one hypervisor to
another. Windows VMs are particularly concerned by this, as the OS is very
picky about any changes of hardware. This problem may be solved by
installing the MergeIDE.zip utility available from the Internet before exporting
and choosing a hard disk type of *IDE* before booting the imported Windows VM.

Finally there is the question of paravirtualized drivers, which improve the
speed of the emulated system and are specific to the hypervisor.
GNU/Linux and other free Unix OSes have all the necessary drivers installed by
default and you can switch to the paravirtualized drivers right after importing
the VM. For Windows VMs, you need to install the Windows paravirtualized
drivers by yourself.

GNU/Linux and other free Unix can usually be imported without hassle. Note
that we cannot guarantee a successful import/export of Windows VMs in all
cases due to the problems above.

Step-by-step example of a Windows OVF import
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Microsoft provides
https://developer.microsoft.com/en-us/windows/downloads/virtual-machines/[Virtual Machines downloads]
 to get started with Windows development.We are going to use one of these
to demonstrate the OVF import feature.

Download the Virtual Machine zip
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

After getting informed about the user agreement, choose the _Windows 10
Enterprise (Evaluation - Build)_ for the VMware platform, and download the zip.

Extract the disk image from the zip
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Using the `unzip` utility or any archiver of your choice, unpack the zip,
and copy via ssh/scp the ovf and vmdk files to your {pve} host.

Import the Virtual Machine
^^^^^^^^^^^^^^^^^^^^^^^^^^

This will create a new virtual machine, using cores, memory and
VM name as read from the OVF manifest, and import the disks to the +local-lvm+
 storage. You have to configure the network manually.

 qm importovf 999 WinDev1709Eval.ovf local-lvm

The VM is ready to be started.

Adding an external disk image to a Virtual Machine
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also add an existing disk image to a VM, either coming from a
foreign hypervisor, or one that you created yourself.

Suppose you created a Debian/Ubuntu disk image with the 'vmdebootstrap' tool:

 vmdebootstrap --verbose \
  --size 10GiB --serial-console \
  --grub --no-extlinux \
  --package openssh-server \
  --package avahi-daemon \
  --package qemu-guest-agent \
  --hostname vm600 --enable-dhcp \
  --customize=./copy_pub_ssh.sh \
  --sparse --image vm600.raw

You can now create a new target VM for this image.

 qm create 600 --net0 virtio,bridge=vmbr0 --name vm600 --serial0 socket \
   --bootdisk scsi0 --scsihw virtio-scsi-pci --ostype l26

Add the disk image as +unused0+ to the VM, using the storage +pvedir+:

 qm importdisk 600 vm600.raw pvedir

Finally attach the unused disk to the SCSI controller of the VM:

 qm set 600 --scsi0 pvedir:600/vm-600-disk-1.raw

The VM is ready to be started.


ifndef::wiki[]
include::qm-cloud-init.adoc[]
endif::wiki[]

ifndef::wiki[]
include::qm-pci-passthrough.adoc[]
endif::wiki[]

Hookscripts
-----------

You can add a hook script to VMs with the config property `hookscript`.

 qm set 100 -hookscript local:snippets/hookscript.pl

It will be called during various phases of the guests lifetime.
For an example and documentation see the example script under
`/usr/share/pve-docs/examples/guest-example-hookscript.pl`.

[[qm_hibernate]]
Hibernation
-----------

You can suspend a VM to disk with the GUI option `Hibernate` or with

 qm suspend ID --todisk

That means that the current content of the memory will be saved onto disk
and the VM gets stopped. On the next start, the memory content will be
loaded and the VM can continue where it was left off.

[[qm_vmstatestorage]]
.State storage selection
If no target storage for the memory is given, it will be automatically
chosen, the first of:

1. The storage `vmstatestorage` from the VM config.
2. The first shared storage from any VM disk.
3. The first non-shared storage from any VM disk.
4. The storage `local` as a fallback.

Managing Virtual Machines with `qm`
------------------------------------

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
create and delete virtual disks.

CLI Usage Examples
~~~~~~~~~~~~~~~~~~

Using an iso file uploaded on the 'local' storage, create a VM
with a 4 GB IDE disk on the 'local-lvm' storage

 qm create 300 -ide0 local-lvm:4 -net0 e1000 -cdrom local:iso/proxmox-mailgateway_2.1.iso

Start the new VM

 qm start 300

Send a shutdown request, then wait until the VM is stopped.

 qm shutdown 300 && qm wait 300

Same as above, but only wait for 40 seconds.

 qm shutdown 300 && qm wait 300 -timeout 40


[[qm_configuration]]
Configuration
-------------

VM configuration files are stored inside the Proxmox cluster file
system, and can be accessed at `/etc/pve/qemu-server/<VMID>.conf`.
Like other files stored inside `/etc/pve/`, they get automatically
replicated to all other cluster nodes.

NOTE: VMIDs < 100 are reserved for internal purposes, and VMIDs need to be
unique cluster wide.

.Example VM Configuration
----
cores: 1
sockets: 1
memory: 512
name: webmail
ostype: l26
bootdisk: virtio0
net0: e1000=EE:D2:28:5F:B6:3E,bridge=vmbr0
virtio0: local:vm-100-disk-1,size=32G
----

Those configuration files are simple text files, and you can edit them
using a normal text editor (`vi`, `nano`, ...). This is sometimes
useful to do small corrections, but keep in mind that you need to
restart the VM to apply such changes.

For that reason, it is usually better to use the `qm` command to
generate and modify those files, or do the whole thing using the GUI.
Our toolkit is smart enough to instantaneously apply most changes to
running VM. This feature is called "hot plug", and there is no
need to restart the VM in that case.


File Format
~~~~~~~~~~~

VM configuration files use a simple colon separated key/value
format. Each line has the following format:

-----
# this is a comment
OPTION: value
-----

Blank lines in those files are ignored, and lines starting with a `#`
character are treated as comments and are also ignored.


[[qm_snapshots]]
Snapshots
~~~~~~~~~

When you create a snapshot, `qm` stores the configuration at snapshot
time into a separate snapshot section within the same configuration
file. For example, after creating a snapshot called ``testsnapshot'',
your configuration file will look like this:

.VM configuration with snapshot
----
memory: 512
swap: 512
parent: testsnaphot
...

[testsnaphot]
memory: 512
swap: 512
snaptime: 1457170803
...
----

There are a few snapshot related properties like `parent` and
`snaptime`. The `parent` property is used to store the parent/child
relationship between snapshots. `snaptime` is the snapshot creation
time stamp (Unix epoch).

You can optionally save the memory of a running VM with the option `vmstate`.
For details about how the target storage gets chosen for the VM state, see
xref:qm_vmstatestorage[State storage selection] in the chapter
xref:qm_hibernate[Hibernation].

[[qm_options]]
Options
~~~~~~~

include::qm.conf.5-opts.adoc[]


Locks
-----

Online migrations, snapshots and backups (`vzdump`) set a lock to
prevent incompatible concurrent actions on the affected VMs. Sometimes
you need to remove such a lock manually (e.g., after a power failure).

 qm unlock <vmid>

CAUTION: Only do that if you are sure the action which set the lock is
no longer running.


ifdef::wiki[]

See Also
~~~~~~~~

* link:/wiki/Cloud-Init_Support[Cloud-Init Support]

endif::wiki[]


ifdef::manvolnum[]

Files
------

`/etc/pve/qemu-server/<VMID>.conf`::

Configuration file for the VM '<VMID>'.


include::pve-copyright.adoc[]
endif::manvolnum[]