ifdef::manvolnum[] PVE({manvolnum}) ================ include::attributes.txt[] NAME ---- qm - Qemu/KVM Virtual Machine Manager SYNOPSYS -------- include::qm.1-synopsis.adoc[] DESCRIPTION ----------- endif::manvolnum[] ifndef::manvolnum[] Qemu/KVM Virtual Machines ========================= include::attributes.txt[] 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 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. 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] 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. .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). 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/.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 ifdef::manvolnum[] include::pve-copyright.adoc[] endif::manvolnum[]