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
+* 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 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.
+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+which has been superseded in features by the Virtio SCSI Controller.
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
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*.
+.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.
+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.
+
+Network Device
+~~~~~~~~~~~~~~
+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).
+
+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.
+
+
Managing Virtual Machines with 'qm'
------------------------------------
projects / pve-docs.git / blobdiff