+// 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 use a 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.
+
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