document ballooning for fixed memory size

[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 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]


[[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="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="gui-create-vm-os.png"]

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.


[[qm_hard_disk]]
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.
+
A SCSI controller of type _Virtio_ 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.

* The *Virtio* controller, also called virtio-blk to distinguish from
the Virtio SCSI controller, is an older type of paravirtualized controller
which has been superseded in features by the Virtio SCSI Controller.

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

.IO Thread
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.


[[qm_cpu]]
CPU
~~~

[thumbnail="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 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.


[[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="gui-create-vm-memory-fixed.png"]

When choosing a *fixed size memory* {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 porpuses), simply uncheck
*Ballooning* or set

 balloon: 0

in the configuration.

.Automatic Memory Allocation
[thumbnail="gui-create-vm-memory-dynamic.png", float="left"]

// 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.


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

[thumbnail="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 differents 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 builting router and DHCP server can
provide network access. This built-in DHCP will serve adresses 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.

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 transfered.

//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 spawn 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 eth0 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.


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).


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

In order to properly emulate a computer, QEMU needs to use a firmware.
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 implemenation. 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_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="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)
* *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 60, which means that {pve} will issue a
shutdown request, wait 60s for the machine to be offline, and if after 60s
the machine is still online will notify that the shutdown action failed.

Please note that machines without a Start/Shutdown order parameter will always
start after those where the parameter is set, and this parameter only
makes sense between the machines running locally on a host, and not
cluster-wide.


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

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

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

 qm migrate <vmid> <target>

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.

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="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 need 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 supports 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 the original template while linked clones
exists.
+

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 gets
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.


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


[[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).


[[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::manvolnum[]

Files
------

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

Configuration file for the VM '<VMID>'.


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