cleanup typos / phrasing

[pve-docs.git] / qm.adoc

diff --git a/qm.adoc b/qm.adoc
index 7d2efecd348bb1f7626855102f9b54e19007d491..39825497aa4210b62fab941605226d02da120e45 100644 (file)
--- a/qm.adoc
+++ b/qm.adoc
@@ -130,7 +130,7 @@ 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
 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 Even if this controller has been superseded by more more designs,
+controller. Even if this controller has been superseded by recent designs,

 each and every OS you can think of 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.
 each and every OS you can think of 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.


@@ -154,9 +154,9 @@ _VirtIO SCSI single_ which will allow you to select the *IO Thread* option.
 When selecting _VirtIO SCSI single_ Qemu will create a new controller for
 each disk, instead of adding all disks to the same controller.
 
 When selecting _VirtIO SCSI single_ Qemu will create a new controller for
 each disk, instead of adding all disks to the same controller.
 

-* 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-which has been superseded in features by the Virtio SCSI Controller.
+* The *VirtIO Block* controller, often just called VirtIO or virtio-blk,
+is an older type of paravirtualized controller. It has been superseded by the
+VirtIO SCSI Controller, in terms of features.

 
 [thumbnail="gui-create-vm-hard-disk.png"]
 On each controller you attach a number of emulated hard disks, which are backed
 
 [thumbnail="gui-create-vm-hard-disk.png"]
 On each controller you attach a number of emulated hard disks, which are backed


@@ -170,9 +170,9 @@ either the *raw disk image format* or the *QEMU image format*.
   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
   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
+ format does not support thin provisioning or snapshots by itself, requiring
+ cooperation from the storage layer for these tasks. It may, however, be up to
+ 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.
  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.


@@ -219,9 +219,9 @@ 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.
 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.
+However some software licenses depend on the number of sockets a machine has,
+in that case it makes sense to set the number of sockets to what the license
+allows you.

 
 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.
 
 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.


@@ -230,14 +230,58 @@ 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.
 
 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 (i.e.
-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.
+NOTE: It is perfectly safe if the _overall_ number of cores of all your VMs
+is greater than the number of cores on the server (e.g., 4 VMs with each 4
+cores on a machine with only 8 cores). 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 from assigning more virtual CPU cores than physically available, as this will
+only bring the performance down due to the cost of context switches.
+
+[[qm_cpu_resource_limits]]
+Resource Limits
+^^^^^^^^^^^^^^^
+
+In addition to the number of virtual cores, you can configure how much resources
+a VM can get in relation to the host CPU time and also in relation to other
+VMs.
+With the *cpulimit* (`Host CPU Time') option you can limit how much CPU time the
+whole VM can use on the host. It is a floating point value representing CPU
+time in percent, so `1.0` is equal to `100%`, `2.5` to `250%` and so on. If a
+single process would fully use one single core it would have `100%` CPU Time
+usage. If a VM with four cores utilizes all its cores fully it would
+theoretically use `400%`. In reality the usage may be even a bit higher as Qemu
+can have additional threads for VM peripherals besides the vCPU core ones.
+This setting can be useful if a VM should have multiple vCPUs, as it runs a few
+processes in parallel, but the VM as a whole should not be able to run all
+vCPUs at 100% at the same time. Using a specific example: lets say we have a VM
+which would profit from having 8 vCPUs, but at no time all of those 8 cores
+should run at full load - as this would make the server so overloaded that
+other VMs and CTs would get to less CPU. So, we set the *cpulimit* limit to
+`4.0` (=400%). If all cores do the same heavy work they would all get 50% of a
+real host cores CPU time. But, if only 4 would do work they could still get
+almost 100% of a real core each.
+
+NOTE: VMs can, depending on their configuration, use additional threads e.g.,
+for networking or IO operations but also live migration. Thus a VM can show up
+to use more CPU time than just its virtual CPUs could use. To ensure that a VM
+never uses more CPU time than virtual CPUs assigned set the *cpulimit* setting
+to the same value as the total core count.
+
+The second CPU resource limiting setting, *cpuunits* (nowadays often called CPU
+shares or CPU weight), controls how much CPU time a VM gets in regards to other
+VMs running.  It is a relative weight which defaults to `1024`, if you increase
+this for a VM it will be prioritized by the scheduler in comparison to other
+VMs with lower weight. E.g., if VM 100 has set the default 1024 and VM 200 was
+changed to `2048`, the latter VM 200 would receive twice the CPU bandwidth than
+the first VM 100.
+
+For more information see `man systemd.resource-control`, here `CPUQuota`
+corresponds to `cpulimit` and `CPUShares` corresponds to our `cpuunits`
+setting, visit its Notes section for references and implementation details.
+
+CPU Type
+^^^^^^^^

 
 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
 
 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


@@ -256,22 +300,60 @@ 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
 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.
+the kvm64 default. If you don’t care about live migration or have a homogeneous
+cluster where all nodes have the same CPU, set the CPU type to host, as in
+theory this will give your guests maximum performance.
+
+NUMA
+^^^^
+You can also optionally emulate a *NUMA*
+footnote:[https://en.wikipedia.org/wiki/Non-uniform_memory_access] 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
 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.
+will allow proper distribution of the VM resources on the host system.
+This option is also required to hot-plug cores or RAM in 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.
 
 
 If the NUMA option is used, it is recommended to set the number of sockets to
 the number of sockets of the host system.
 

+vCPU hot-plug
+^^^^^^^^^^^^^
+
+Modern operating systems introduced the capability to hot-plug and, to a
+certain extent, hot-unplug CPUs in a running systems. Virtualisation allows us
+to avoid a lot of the (physical) problems real hardware can cause in such
+scenarios.
+Still, this is a rather new and complicated feature, so its use should be
+restricted to cases where its absolutely needed. Most of the functionality can
+be replicated with other, well tested and less complicated, features, see
+xref:qm_cpu_resource_limits[Resource Limits].
+
+In {pve} the maximal number of plugged CPUs is always `cores * sockets`.
+To start a VM with less than this total core count of CPUs you may use the
+*vpus* setting, it denotes how many vCPUs should be plugged in at VM start.
+
+Currently only this feature is only supported on Linux, a kernel newer than 3.10
+is needed, a kernel newer than 4.7 is recommended.
+
+You can use a udev rule as follow to automatically set new CPUs as online in
+the guest:
+
+----
+SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ATTR{online}="1"
+----
+
+Save this under /etc/udev/rules.d/ as a file ending in `.rules`.
+
+Note: CPU hot-remove is machine dependent and requires guest cooperation.
+The deletion command does not guarantee CPU removal to actually happen,
+typically it's a request forwarded to guest using target dependent mechanism,
+e.g., ACPI on x86/amd64.
+

 
 [[qm_memory]]
 Memory
 
 [[qm_memory]]
 Memory