add paragraph for IO Thread option

[pve-docs.git] / qm.adoc

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NAME
----

qm - Qemu/KVM Virtual Machine Manager


SYNOPSYS
--------

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

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. +
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*.
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.

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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