en-US/over-provisioning-service-offering-limits.xml

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<section id="over-provisioning-service-offering-limits">
    <title>Over-Provisioning and Service Offering Limits</title>
  <para>(Supported for XenServer, KVM, and VMware)</para>
  <para>CPU and memory (RAM) over-provisioning factors can be set for each cluster to change the
    number of VMs that can run on each host in the cluster. This helps optimize the use of
    resources. By increasing the over-provisioning ratio, more resource capacity will be used. If
    the ratio is set to 1, no over-provisioning is done.</para>
  <para>The administrator can also set global default over-provisioning ratios
    in the cpu.overprovisioning.factor and mem.overprovisioning.factor global configuration variables.
    The default value of these variables is 1: over-provisioning is turned off by default.
  </para>
  <para>Over-provisioning ratios are dynamically substituted in &PRODUCT;'s capacity
    calculations. For example: </para>
  <para>Capacity = 2 GB</para>
  <para>Over-provisioning factor = 2</para>
  <para>Capacity after over-provisioning = 4 GB</para>
  <para>With this configuration, suppose you deploy 3 VMs of 1 GB each:</para>
  <para>Used = 3 GB</para>
  <para>Free = 1 GB</para>
  <para>The administrator can specify a memory over-provisioning ratio, and can specify both CPU and
    memory over-provisioning ratios on a per-cluster basis.</para>
  <para>In any given cloud, the optimum number of VMs for each host is affected by such things as
    the hypervisor, storage, and hardware configuration. These may be different for each cluster in
    the same cloud. A single global over-provisioning setting can not provide the best utilization
    for all the different clusters in the cloud. It has to be set for the lowest common denominator.
    The per-cluster setting provides a finer granularity for better utilization of resources, no
    matter where the &PRODUCT; placement algorithm decides to place a VM.</para>
  <para>The overprovisioning settings can be used along with dedicated resources (assigning a
    specific cluster to an account) to effectively offer different levels of service to
    different accounts. For example, an account paying for a more expensive level of service
    could be assigned to a dedicated cluster with an over-provisioning ratio of 1, and a
    lower-paying account to a cluster with a ratio of 2.</para>
  <para>When a new host is added to a cluster, &PRODUCT; will assume the host has the
    capability to perform the CPU and RAM over-provisioning which is configured for that
    cluster. It is up to the administrator to be sure the host is actually suitable for the
    level of over-provisioning which has been set.</para>
  <section id="overcommit-limitations">
    <title>Limitations on Over-Provisioning in XenServer and KVM</title>
    <itemizedlist>
      <listitem><para>In XenServer, due to a constraint of this hypervisor, you can not use an
        over-provisioning factor greater than 4.</para></listitem>
      <listitem><para>The KVM hypervisor can not manage memory allocation to VMs dynamically.
        &PRODUCT; sets the minimum and maximum amount of memory that a VM can use.
        The hypervisor adjusts the memory within the set limits based on the memory contention.</para></listitem>
    </itemizedlist>    
  </section>
  <section id="overcommit-prerequisites">
    <title>Requirements for Over-Provisioning</title>
    <para>Several prerequisites are required in order for over-provisioning to function
      properly. The feature is dependent on the OS type, hypervisor capabilities, and certain
      scripts. It is the administrator's responsibility to ensure that these requirements are
      met.</para>
    <section id="balloon-driver">
      <title>Balloon Driver</title>
      <para>All VMs should have a balloon driver installed in them. The hypervisor
        communicates with the balloon driver to free up and make the memory available to a
        VM.</para>
      <formalpara>
        <title>XenServer</title>
        <para>The balloon driver can be found as a part of xen pv or PVHVM drivers. The xen
          pvhvm drivers are included in upstream linux kernels 2.6.36+.</para>
      </formalpara>
      <formalpara>
        <title>VMware</title>
        <para>The balloon driver can be found as a part of the VMware tools. All the VMs that
          are deployed in a over-provisioned cluster should have the VMware tools
          installed.</para>
      </formalpara>
      <formalpara>
        <title>KVM</title>
        <para>All VMs are required to support the virtio drivers. These drivers are installed
          in all Linux kernel versions 2.6.25 and greater. The administrator must set
          CONFIG_VIRTIO_BALLOON=y in the virtio configuration. </para>
      </formalpara>
    </section>
    <section id="memory-ballooning">
      <title>Hypervisor capabilities</title>
      <para>The hypervisor must be capable of using the memory ballooning.</para>
      <formalpara>
        <title>XenServer</title>
        <para>The DMC (Dynamic Memory Control) capability of the hypervisor should be enabled.
          Only XenServer Advanced and above versions have this feature.</para>
      </formalpara>
      <formalpara>
        <title>VMware, KVM</title>
        <para>Memory ballooning is supported by default.</para>
      </formalpara>
    </section>
  </section>
  <section id="create-overcommit">
    <title>Setting Over-Provisioning Ratios</title>
    <para>There are two ways the root admin can set CPU and RAM over-provisioning ratios. First, the
      global configuration settings cpu.overprovisioning.factor and mem.overprovisioning.factor will
      be applied when a new cluster is created. Later, the ratios can be modified for an existing
      cluster.</para>
    <para>Only VMs deployed after the change are affected by the new setting.
      If you want VMs deployed before the change to adopt the new over-provisioning ratio,
      you must stop and restart the VMs.
      When this is done, &PRODUCT; recalculates or scales the used and 
      reserved capacities based on the new over-provisioning ratios,
      to ensure that &PRODUCT; is correctly tracking the amount of free capacity.</para>
    <note><para>It is safer not to deploy additional new VMs while the capacity recalculation is underway, in
        case the new values for available capacity are not high enough to accommodate the new VMs.
        Just wait for the new used/available values to become available, to be sure there is room
        for all the new VMs you want.</para></note>
    <para>To change the over-provisioning ratios for an existing cluster:</para>
    <orderedlist>
      <listitem>
        <para>Log in as administrator to the &PRODUCT; UI.</para>
      </listitem>
      <listitem>
        <para>In the left navigation bar, click Infrastructure.</para>
      </listitem>
      <listitem>
        <para>Under Clusters, click View All.</para>
      </listitem>
      <listitem>
        <para>Select the cluster you want to work with, and click the Edit button.</para>
      </listitem>
      <listitem>
        <para>Fill in your desired over-provisioning multipliers in the fields CPU overcommit
          ratio and RAM overcommit ratio. The value which is intially shown in these
          fields is the default value inherited from the global configuration settings.
        </para>
        <note>
          <para>In XenServer, due to a constraint of this hypervisor, you can not use an
            over-provisioning factor greater than 4.</para>
        </note>
      </listitem>
    </orderedlist>
  </section>
  <section id="op-service-offering-limits">
    <title>Service Offering Limits and Over-Provisioning</title>
    <para>Service offering   limits (e.g. 1 GHz, 1 core) are strictly enforced for core count.  For example, a guest with a service offering of one core will have only one core available to it regardless of other activity on the Host.  </para>
    <para>Service offering limits for gigahertz are enforced only in the presence of contention for CPU resources.  For example, suppose that a guest was created with a service offering of 1 GHz on a Host that has 2 GHz cores, and that guest is the only guest running on the Host.  The guest will have the full 2 GHz available to it.  When multiple guests are attempting to use the CPU a weighting factor is used to schedule CPU resources.  The weight is based on the clock speed in the service offering.  Guests receive a CPU allocation that is proportionate to the GHz in the service offering.   For example, a guest created from a 2 GHz service offering will receive twice the CPU allocation as a guest created from a 1 GHz service offering. &PRODUCT; does not perform memory over-provisioning.</para>
  </section>
</section>