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.
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
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).
+
Managing Virtual Machines with 'qm'
------------------------------------
projects / pve-docs.git / blobdiff