pve-docs/qm.adoc

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NAME
----

qm - Qemu/KVM Virtual Machine Manager


SYNOPSYS
--------

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DESCRIPTION
-----------
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Qemu/KVM Virtual Machines
=========================
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// 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 opensource 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 emulates 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 use 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 presente
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]

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.

General Settings
~~~~~~~~~~~~~~~~
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

OS Settings
~~~~~~~~~~~
When creating a 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.

Hard Disk
~~~~~~~~~
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 more more designs,
each and every OS you can think 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.

* The *Virtio* controller is a generic paravirtualized controller, and is the
recommended setting if you aim for performance. To use this controller, the OS
need to have special drivers which may be included in your installation ISO or
not. Linux distributions have support for the Virtio controller since 2010, and
FreeBSD since 2014. For Windows OSes, you need to provide an extra iso
containing the Virtio drivers during the installation.
// see: https://pve.proxmox.com/wiki/Paravirtualized_Block_Drivers_for_Windows#During_windows_installation.
You can connect up to 16 devices on this controller.

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, 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 do not support thin provisioning or snapshotting by itself, requiring
 cooperation from the storage layer for these tasks. It is however 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.

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 your storage supports _thin provisioning_ (see the storage chapter in the
{pve} guide), and your VM has a *SCSI* controller you can activate the *Discard*
option on the hard disks connected to that controller. With *Discard* enabled,
when the filesystem of a VM marks blocks as unused after removing files, the
emulated SCSI controller will relay this information to the storage, which will
then shrink the disk image accordingly.

The option *IO Thread* can only be enabled when using a disk with the *VirtIO* controller,
or with the *SCSI* controller, when the emulated controller type is  *VirtIO SCSI*.
With this enabled, Qemu uses one thread per disk, instead of one thread for all,
so it should increase performance when using multiple disks.
Note that backups do not currently work with *IO Thread* enabled.

CPU
~~~
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 is licensed depending on the number of sockets you have in
your machine, in that case it makes sense to set the number of of sockets to
what the license allows you, and increase the number of cores. +
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 to set the _overall_ number of total cores in all
your VMs to be greater than the number of of cores you have on your server (ie.
4 VMs with each 4 Total cores running in a 8 core machine is OK) 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 to allocate on a _single_ machine more vcpus than
physically available, as this will only bring the performance down due to the
cost of context switches.

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, set the CPU type to
host, as in theory this will give your guests maximum performance.

You can also optionally emulate a *NUMA* 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 in {pve} to allow hotplugging of cores and RAM to a VM.

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

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. 

When choosing a *fixed size memory* {pve} will simply allocate what you
specify to your VM.

// see autoballoon() in pvestatd.pm
When choosing to *automatically allocate 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 becoming short 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 shoud take. Suppose for instance you have four VMs, three of them
running a 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 curring 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 RAMs to your VMs, a good rule of thumb is always to leave 1GB
of RAM available to the host.

USB Passthrough
~~~~~~~~~~~~~~~
There are two different types of USB passthrough devices:

* Host USB passtrough
* 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 ist 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).

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
~~~~~~~~~~~~~~~~~~

Create a new VM with 4 GB IDE disk.

 qm create 300 -ide0 4 -net0 e1000 -cdrom 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

Configuration
-------------

All configuration files consists of lines in the form

 PARAMETER: value

Configuration files are stored inside the Proxmox cluster file
system, and can be accessed at '/etc/pve/qemu-server/<VMID>.conf'.

Options
~~~~~~~

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


Locks
-----

Online migrations 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>


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