Flavors

Flavors

In OpenStack, flavors define the compute, memory, and storage capacity of nova computing instances. To put it simply, a flavor is an available hardware configuration for a server. It defines the size of a virtual server that can be launched.

Note

Flavors can also determine on which compute host a flavor can be used to launch an instance. For information about customizing flavors, refer to Manage Flavors.

Overview

A flavor consists of the following parameters:

Flavor ID
Unique ID (integer or UUID) for the new flavor. This property is required. If specifying ‘auto’, a UUID will be automatically generated.
Name

Name for the new flavor. This property is required.

Historically, names were given a format XX.SIZE_NAME. These are typically not required, though some third party tools may rely on it.

VCPUs
Number of virtual CPUs to use. This property is required.
Memory MB
Amount of RAM to use (in megabytes). This property is required.
Root Disk GB

Amount of disk space (in gigabytes) to use for the root (/) partition. This property is required.

The root disk is an ephemeral disk that the base image is copied into. When booting from a persistent volume it is not used. The 0 size is a special case which uses the native base image size as the size of the ephemeral root volume. However, in this case the filter scheduler cannot select the compute host based on the virtual image size. As a result, 0 should only be used for volume booted instances or for testing purposes.

Ephemeral Disk GB

Amount of disk space (in gigabytes) to use for the ephemeral partition. This property is optional. If unspecified, the value is 0 by default.

Ephemeral disks offer machine local disk storage linked to the lifecycle of a VM instance. When a VM is terminated, all data on the ephemeral disk is lost. Ephemeral disks are not included in any snapshots.

Swap
Amount of swap space (in megabytes) to use. This property is optional. If unspecified, the value is 0 by default.
RXTX Factor

The receive/transmit factor of any network ports on the instance. This property is optional. If unspecified, the value is 1.0 by default.

Note

This property only applies if using the xen compute driver with the nova-network network driver. It will likely be deprecated in a future release. neutron users should refer to the neutron QoS documentation.

Is Public

Boolean value that defines whether the flavor is available to all users or private to the project it was created in. This property is optional. In unspecified, the value is True by default.

By default, a flavor is public and available to all projects. Private flavors are only accessible to those on the access list for a given project and are invisible to other projects.

Extra Specs

Key and value pairs that define on which compute nodes a flavor can run. These are optional.

Extra specs are generally used as scheduler hints for more advanced instance configuration. The key-value pairs used must correspond to well-known options. For more information on the standardized extra specs available, see below

Extra Specs

Todo

A lot of these need investigation - for example, I can find no reference to the cpu_shares_level option outside of documentation and (possibly) useless tests. We should assess which drivers each option actually apply to.

CPU limits

You can configure the CPU limits with control parameters. For example, to configure the I/O limit, use:

$ openstack flavor set FLAVOR-NAME \
    --property quota:read_bytes_sec=10240000 \
    --property quota:write_bytes_sec=10240000

Use these optional parameters to control weight shares, enforcement intervals for runtime quotas, and a quota for maximum allowed bandwidth:

  • cpu_shares: Specifies the proportional weighted share for the domain. If this element is omitted, the service defaults to the OS provided defaults. There is no unit for the value; it is a relative measure based on the setting of other VMs. For example, a VM configured with value 2048 gets twice as much CPU time as a VM configured with value 1024.

  • cpu_shares_level: On VMware, specifies the allocation level. Can be custom, high, normal, or low. If you choose custom, set the number of shares using cpu_shares_share.

  • cpu_period: Specifies the enforcement interval (unit: microseconds) for QEMU and LXC hypervisors. Within a period, each VCPU of the domain is not allowed to consume more than the quota worth of runtime. The value should be in range [1000, 1000000]. A period with value 0 means no value.

  • cpu_limit: Specifies the upper limit for VMware machine CPU allocation in MHz. This parameter ensures that a machine never uses more than the defined amount of CPU time. It can be used to enforce a limit on the machine’s CPU performance.

  • cpu_reservation: Specifies the guaranteed minimum CPU reservation in MHz for VMware. This means that if needed, the machine will definitely get allocated the reserved amount of CPU cycles.

  • cpu_quota: Specifies the maximum allowed bandwidth (unit: microseconds). A domain with a negative-value quota indicates that the domain has infinite bandwidth, which means that it is not bandwidth controlled. The value should be in range [1000, 18446744073709551] or less than 0. A quota with value 0 means no value. You can use this feature to ensure that all vCPUs run at the same speed. For example:

    $ openstack flavor set FLAVOR-NAME \
        --property quota:cpu_quota=10000 \
        --property quota:cpu_period=20000
    

    In this example, an instance of FLAVOR-NAME can only consume a maximum of 50% CPU of a physical CPU computing capability.

Memory limits

For VMware, you can configure the memory limits with control parameters.

Use these optional parameters to limit the memory allocation, guarantee minimum memory reservation, and to specify shares used in case of resource contention:

  • memory_limit: Specifies the upper limit for VMware machine memory allocation in MB. The utilization of a virtual machine will not exceed this limit, even if there are available resources. This is typically used to ensure a consistent performance of virtual machines independent of available resources.

  • memory_reservation: Specifies the guaranteed minimum memory reservation in MB for VMware. This means the specified amount of memory will definitely be allocated to the machine.

  • memory_shares_level: On VMware, specifies the allocation level. This can be custom, high, normal or low. If you choose custom, set the number of shares using memory_shares_share.

  • memory_shares_share: Specifies the number of shares allocated in the event that custom is used. There is no unit for this value. It is a relative measure based on the settings for other VMs. For example:

    $ openstack flavor set FLAVOR-NAME \
        --property quota:memory_shares_level=custom \
        --property quota:memory_shares_share=15
    
Disk I/O limits

For VMware, you can configure the resource limits for disk with control parameters.

Use these optional parameters to limit the disk utilization, guarantee disk allocation, and to specify shares used in case of resource contention. This allows the VMware driver to enable disk allocations for the running instance.

  • disk_io_limit: Specifies the upper limit for disk utilization in I/O per second. The utilization of a virtual machine will not exceed this limit, even if there are available resources. The default value is -1 which indicates unlimited usage.

  • disk_io_reservation: Specifies the guaranteed minimum disk allocation in terms of Input/output Operations Per Second (IOPS).

  • disk_io_shares_level: Specifies the allocation level. This can be custom, high, normal or low. If you choose custom, set the number of shares using disk_io_shares_share.

  • disk_io_shares_share: Specifies the number of shares allocated in the event that custom is used. When there is resource contention, this value is used to determine the resource allocation.

    The example below sets the disk_io_reservation to 2000 IOPS.

    $ openstack flavor set FLAVOR-NAME \
        --property quota:disk_io_reservation=2000
    
Disk tuning

Using disk I/O quotas, you can set maximum disk write to 10 MB per second for a VM user. For example:

$ openstack flavor set FLAVOR-NAME \
    --property quota:disk_write_bytes_sec=10485760

The disk I/O options are:

  • disk_read_bytes_sec
  • disk_read_iops_sec
  • disk_write_bytes_sec
  • disk_write_iops_sec
  • disk_total_bytes_sec
  • disk_total_iops_sec
Bandwidth I/O

The vif I/O options are:

  • vif_inbound_average
  • vif_inbound_burst
  • vif_inbound_peak
  • vif_outbound_average
  • vif_outbound_burst
  • vif_outbound_peak

Incoming and outgoing traffic can be shaped independently. The bandwidth element can have at most, one inbound and at most, one outbound child element. If you leave any of these child elements out, no quality of service (QoS) is applied on that traffic direction. So, if you want to shape only the network’s incoming traffic, use inbound only (and vice versa). Each element has one mandatory attribute average, which specifies the average bit rate on the interface being shaped.

There are also two optional attributes (integer): peak, which specifies the maximum rate at which a bridge can send data (kilobytes/second), and burst, the amount of bytes that can be burst at peak speed (kilobytes). The rate is shared equally within domains connected to the network.

The example below sets network traffic bandwidth limits for existing flavor as follows:

  • Outbound traffic:
    • average: 262 Mbps (32768 kilobytes/second)
    • peak: 524 Mbps (65536 kilobytes/second)
    • burst: 65536 kilobytes
  • Inbound traffic:
    • average: 262 Mbps (32768 kilobytes/second)
    • peak: 524 Mbps (65536 kilobytes/second)
    • burst: 65536 kilobytes
$ openstack flavor set FLAVOR-NAME \
    --property quota:vif_outbound_average=32768 \
    --property quota:vif_outbound_peak=65536 \
    --property quota:vif_outbound_burst=65536 \
    --property quota:vif_inbound_average=32768 \
    --property quota:vif_inbound_peak=65536 \
    --property quota:vif_inbound_burst=65536

Note

All the speed limit values in above example are specified in kilobytes/second. And burst values are in kilobytes. Values were converted using Data rate units on Wikipedia.

Watchdog behavior

For the libvirt driver, you can enable and set the behavior of a virtual hardware watchdog device for each flavor. Watchdog devices keep an eye on the guest server, and carry out the configured action, if the server hangs. The watchdog uses the i6300esb device (emulating a PCI Intel 6300ESB). If hw:watchdog_action is not specified, the watchdog is disabled.

To set the behavior, use:

$ openstack flavor set FLAVOR-NAME --property hw:watchdog_action=ACTION

Valid ACTION values are:

  • disabled: (default) The device is not attached.
  • reset: Forcefully reset the guest.
  • poweroff: Forcefully power off the guest.
  • pause: Pause the guest.
  • none: Only enable the watchdog; do nothing if the server hangs.

Note

Watchdog behavior set using a specific image’s properties will override behavior set using flavors.

Random-number generator

If a random-number generator device has been added to the instance through its image properties, the device can be enabled and configured using:

$ openstack flavor set FLAVOR-NAME \
    --property hw_rng:allowed=True \
    --property hw_rng:rate_bytes=RATE-BYTES \
    --property hw_rng:rate_period=RATE-PERIOD

Where:

  • RATE-BYTES: (integer) Allowed amount of bytes that the guest can read from the host’s entropy per period.
  • RATE-PERIOD: (integer) Duration of the read period in seconds.
CPU topology

For the libvirt driver, you can define the topology of the processors in the virtual machine using properties. The properties with max limit the number that can be selected by the user with image properties.

$ openstack flavor set FLAVOR-NAME \
    --property hw:cpu_sockets=FLAVOR-SOCKETS \
    --property hw:cpu_cores=FLAVOR-CORES \
    --property hw:cpu_threads=FLAVOR-THREADS \
    --property hw:cpu_max_sockets=FLAVOR-SOCKETS \
    --property hw:cpu_max_cores=FLAVOR-CORES \
    --property hw:cpu_max_threads=FLAVOR-THREADS

Where:

  • FLAVOR-SOCKETS: (integer) The number of sockets for the guest VM. By default, this is set to the number of vCPUs requested.
  • FLAVOR-CORES: (integer) The number of cores per socket for the guest VM. By default, this is set to 1.
  • FLAVOR-THREADS: (integer) The number of threads per core for the guest VM. By default, this is set to 1.
CPU pinning policy

For the libvirt driver, you can pin the virtual CPUs (vCPUs) of instances to the host’s physical CPU cores (pCPUs) using properties. You can further refine this by stating how hardware CPU threads in a simultaneous multithreading-based (SMT) architecture be used. These configurations will result in improved per-instance determinism and performance.

Note

SMT-based architectures include Intel processors with Hyper-Threading technology. In these architectures, processor cores share a number of components with one or more other cores. Cores in such architectures are commonly referred to as hardware threads, while the cores that a given core share components with are known as thread siblings.

Note

Host aggregates should be used to separate these pinned instances from unpinned instances as the latter will not respect the resourcing requirements of the former.

$ openstack flavor set FLAVOR-NAME \
    --property hw:cpu_policy=CPU-POLICY \
    --property hw:cpu_thread_policy=CPU-THREAD-POLICY

Valid CPU-POLICY values are:

  • shared: (default) The guest vCPUs will be allowed to freely float across host pCPUs, albeit potentially constrained by NUMA policy.
  • dedicated: The guest vCPUs will be strictly pinned to a set of host pCPUs. In the absence of an explicit vCPU topology request, the drivers typically expose all vCPUs as sockets with one core and one thread. When strict CPU pinning is in effect the guest CPU topology will be setup to match the topology of the CPUs to which it is pinned. This option implies an overcommit ratio of 1.0. For example, if a two vCPU guest is pinned to a single host core with two threads, then the guest will get a topology of one socket, one core, two threads.

Valid CPU-THREAD-POLICY values are:

  • prefer: (default) The host may or may not have an SMT architecture. Where an SMT architecture is present, thread siblings are preferred.
  • isolate: The host must not have an SMT architecture or must emulate a non-SMT architecture. If the host does not have an SMT architecture, each vCPU is placed on a different core as expected. If the host does have an SMT architecture - that is, one or more cores have thread siblings - then each vCPU is placed on a different physical core. No vCPUs from other guests are placed on the same core. All but one thread sibling on each utilized core is therefore guaranteed to be unusable.
  • require: The host must have an SMT architecture. Each vCPU is allocated on thread siblings. If the host does not have an SMT architecture, then it is not used. If the host has an SMT architecture, but not enough cores with free thread siblings are available, then scheduling fails.

Note

The hw:cpu_thread_policy option is only valid if hw:cpu_policy is set to dedicated.

NUMA topology

For the libvirt driver, you can define the host NUMA placement for the instance vCPU threads as well as the allocation of instance vCPUs and memory from the host NUMA nodes. For flavors whose memory and vCPU allocations are larger than the size of NUMA nodes in the compute hosts, the definition of a NUMA topology allows hosts to better utilize NUMA and improve performance of the instance OS.

$ openstack flavor set FLAVOR-NAME \
    --property hw:numa_nodes=FLAVOR-NODES \
    --property hw:numa_cpus.N=FLAVOR-CORES \
    --property hw:numa_mem.N=FLAVOR-MEMORY

Where:

  • FLAVOR-NODES: (integer) The number of host NUMA nodes to restrict execution of instance vCPU threads to. If not specified, the vCPU threads can run on any number of the host NUMA nodes available.
  • N: (integer) The instance NUMA node to apply a given CPU or memory configuration to, where N is in the range 0 to FLAVOR-NODES - 1.
  • FLAVOR-CORES: (comma-separated list of integers) A list of instance vCPUs to map to instance NUMA node N. If not specified, vCPUs are evenly divided among available NUMA nodes.
  • FLAVOR-MEMORY: (integer) The number of MB of instance memory to map to instance NUMA node N. If not specified, memory is evenly divided among available NUMA nodes.

Note

hw:numa_cpus.N and hw:numa_mem.N are only valid if hw:numa_nodes is set. Additionally, they are only required if the instance’s NUMA nodes have an asymmetrical allocation of CPUs and RAM (important for some NFV workloads).

Note

The N parameter is an index of guest NUMA nodes and may not correspond to host NUMA nodes. For example, on a platform with two NUMA nodes, the scheduler may opt to place guest NUMA node 0, as referenced in hw:numa_mem.0 on host NUMA node 1 and vice versa. Similarly, the integers used for FLAVOR-CORES are indexes of guest vCPUs and may not correspond to host CPUs. As such, this feature cannot be used to constrain instances to specific host CPUs or NUMA nodes.

Warning

If the combined values of hw:numa_cpus.N or hw:numa_mem.N are greater than the available number of CPUs or memory respectively, an exception is raised.

CPU real-time policy

For the libvirt driver, you can state that one or more of your instance virtual CPUs (vCPUs), though not all of them, run with a real-time policy. When used on a correctly configured host, this provides stronger guarantees for worst case scheduler latency for vCPUs and is a requirement for certain applications.

Todo

Document the required steps to configure hosts and guests. There are a lot of things necessary, from isolating hosts and configuring the vcpu_pin_set nova configuration option on the host, to choosing a correctly configured guest image.

Important

While most of your instance vCPUs can run with a real-time policy, you must mark at least one vCPU as non-real-time, to be used for both non-real-time guest processes and emulator overhead (housekeeping) processes.

Important

To use this extra spec, you must enable pinned CPUs. Refer to CPU policy for more information.

$ openstack flavor set FLAVOR-NAME \
    --property hw:cpu_realtime=CPU-REALTIME-POLICY \
    --property hw:cpu_realtime_mask=CPU-REALTIME-MASK

Where:

CPU-REALTIME-POLICY (enum):

One of:

  • no: (default) The guest vCPUs will not have a real-time policy
  • yes: The guest vCPUs will have a real-time policy
CPU-REALTIME-MASK (coremask):
A coremask indicating which vCPUs will not have a real-time policy. This should start with a ^. For example, a value of ^0-1 indicates that all vCPUs except vCPUs 0 and 1 will have a real-time policy.

Note

The hw:cpu_realtime_mask option is only valid if hw:cpu_realtime is set to yes.

Emulator threads policy

For the libvirt driver, you can assign a separate pCPU to an instance that will be used for emulator threads, which are emulator processes not directly related to the guest OS. This pCPU will used in addition to the pCPUs used for the guest. This is generally required for use with a real-time workload.

Important

To use this extra spec, you must enable pinned CPUs. Refer to CPU policy for more information.

$ openstack flavor set FLAVOR-NAME \
    --property hw:emulator_threads_policy=THREAD-POLICY

Valid THREAD-POLICY values are:

  • share: (default) The emulator threads float across the pCPUs associated to the guest.
  • isolate: The emulator threads are isolated on a single pCPU.
Large pages allocation

You can configure the size of large pages used to back the VMs.

$ openstack flavor set FLAVOR-NAME \
    --property hw:mem_page_size=PAGE_SIZE

Valid PAGE_SIZE values are:

  • small: (default) The smallest page size is used. Example: 4 KB on x86.
  • large: Only use larger page sizes for guest RAM. Example: either 2 MB or 1 GB on x86.
  • any: It is left up to the compute driver to decide. In this case, the libvirt driver might try to find large pages, but fall back to small pages. Other drivers may choose alternate policies for any.
  • pagesize: (string) An explicit page size can be set if the workload has specific requirements. This value can be an integer value for the page size in KB, or can use any standard suffix. Example: 4KB, 2MB, 2048, 1GB.

Note

Large pages can be enabled for guest RAM without any regard to whether the guest OS will use them or not. If the guest OS chooses not to use huge pages, it will merely see small pages as before. Conversely, if a guest OS does intend to use huge pages, it is very important that the guest RAM be backed by huge pages. Otherwise, the guest OS will not be getting the performance benefit it is expecting.

PCI passthrough

You can assign PCI devices to a guest by specifying them in the flavor.

$ openstack flavor set FLAVOR-NAME \
    --property pci_passthrough:alias=ALIAS:COUNT

Where:

  • ALIAS: (string) The alias which correspond to a particular PCI device class as configured in the nova configuration file (see Configuration Options).
  • COUNT: (integer) The amount of PCI devices of type ALIAS to be assigned to a guest.
Secure Boot

When your Compute services use the Hyper-V hypervisor, you can enable secure boot for Windows and Linux instances. .. code:: console

$ openstack flavor set FLAVOR-NAME
–property os:secure_boot=SECURE_BOOT_OPTION

Valid SECURE_BOOT_OPTION values are:

  • required: Enable Secure Boot for instances running with this flavor.
  • disabled or optional: (default) Disable Secure Boot for instances running with this flavor.
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