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Compute uses the nova-scheduler service to determine how to dispatch compute and volume requests. For example, the nova-scheduler service determines which host a VM should launch on. The term host in the context of filters means a physical node that has a nova-compute service running on it. You can configure the scheduler through a variety of options. Compute is configured with the following default scheduler options: scheduler_driver=nova.scheduler.multi.MultiScheduler compute_scheduler_driver=nova.scheduler.filter_scheduler.FilterScheduler scheduler_available_filters=nova.scheduler.filters.all_filters scheduler_default_filters=RetryFilter,AvailabilityZoneFilter,RamFilter,ComputeFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter By default, the scheduler_driver is configured as a filter scheduler, as described in the next section. In the default configuration, this scheduler considers hosts that meet all the following criteria: Have not been attempted for scheduling purposes (RetryFilter). Are in the requested availability zone (AvailabilityZoneFilter). Have sufficient RAM available (RamFilter). Are capable of servicing the request (ComputeFilter). Satisfy the extra specs associated with the instance type (ComputeCapabilitiesFilter). Satisfy any architecture, hypervisor type, or virtual machine mode properties specified on the instance's image properties. (ImagePropertiesFilter). For information on the volume scheduler, refer the Block Storage section of OpenStack Cloud Administrator Guide for information.

The Filter Scheduler (nova.scheduler.filter_scheduler.FilterScheduler) is the default scheduler for scheduling virtual machine instances. It supports filtering and weighting to make informed decisions on where a new instance should be created.

When the Filter Scheduler receives a request for a resource, it first applies filters to determine which hosts are eligible for consideration when dispatching a resource. Filters are binary: either a host is accepted by the filter, or it is rejected. Hosts that are accepted by the filter are then processed by a different algorithm to decide which hosts to use for that request, described in the Weights section.

The scheduler_available_filters configuration option in nova.conf provides the Compute service with the list of the filters that are used by the scheduler. The default setting specifies all of the filter that are included with the Compute service: scheduler_available_filters=nova.scheduler.filters.all_filters This configuration option can be specified multiple times. For example, if you implemented your own custom filter in Python called myfilter.MyFilter and you wanted to use both the built-in filters and your custom filter, your nova.conf file would contain: scheduler_available_filters=nova.scheduler.filters.all_filters scheduler_available_filters=myfilter.MyFilter The scheduler_default_filters configuration option in nova.conf defines the list of filters that are applied by the nova-scheduler service. As mentioned, the default filters are: scheduler_default_filters=AvailabilityZoneFilter,RamFilter,ComputeFilter The following sections describe the available filters.

Implements blueprint per-aggregate-resource-ratio. AggregateCoreFilter supports per-aggregate cpu_allocation_ratio. If the per-aggregate value is not found, the value falls back to the global setting.

Matches properties defined in an instance type's extra specs against admin-defined properties on a host aggregate. Works with specifications that are unscoped, or are scoped with aggregate_instance_extra_specs. See the host aggregates section for documentation on how to use this filter.

Isolates tenants to specific host aggregates. If a host is in an aggregate that has the metadata key filter_tenant_id it only creates instances from that tenant (or list of tenants). A host can be in different aggregates. If a host does not belong to an aggregate with the metadata key, it can create instances from all tenants.

Implements blueprint per-aggregate-resource-ratio. Supports per-aggregate ram_allocation_ratio. If per-aggregate value is not found, it falls back to the default setting.

This is a no-op filter, it does not eliminate any of the available hosts.

Filters hosts by availability zone. This filter must be enabled for the scheduler to respect availability zones in requests.

Matches properties defined in an instance type's extra specs against compute capabilities. If an extra specs key contains a colon ":", anything before the colon is treated as a namespace, and anything after the colon is treated as the key to be matched. If a namespace is present and is not 'capabilities', it is ignored by this filter. Disable the ComputeCapabilitiesFilter when using a Bare Metal configuration, due to bug 1129485

Passes all hosts that are operational and enabled. In general, this filter should always be enabled.

Only schedule instances on hosts if there are sufficient CPU cores available. If this filter is not set, the scheduler may over provision a host based on cores (for example, the virtual cores running on an instance may exceed the physical cores). This filter can be configured to allow a fixed amount of vCPU overcommitment by using the cpu_allocation_ratio Configuration option in nova.conf. The default setting is: cpu_allocation_ratio=16.0 With this setting, if 8 vCPUs are on a node, the scheduler allows instances up to 128 vCPU to be run on that node. To disallow vCPU overcommitment set: cpu_allocation_ratio=1.0 The Compute API will always return the actual number of CPU cores available on a compute node regardless of the value of the cpu_allocation_ratio configuration key. As a result changes to the cpu_allocation_ratio are not reflected via the command line clients or the dashboard. Changes to this configuration key are only taken into account internally in the scheduler.

Schedule the instance on a different host from a set of instances. To take advantage of this filter, the requester must pass a scheduler hint, using different_host as the key and a list of instance uuids as the value. This filter is the opposite of the SameHostFilter. Using the nova command-line tool, use the --hint flag. For example: $ nova boot --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \ --hint different_host=a0cf03a5-d921-4877-bb5c-86d26cf818e1 \ --hint different_host=8c19174f-4220-44f0-824a-cd1eeef10287 server-1 With the API, use the os:scheduler_hints key. For example:

Only schedule instances on hosts if there is sufficient disk space available for root and ephemeral storage. This filter can be configured to allow a fixed amount of disk overcommitment by using the disk_allocation_ratio Configuration option in nova.conf. The default setting is: disk_allocation_ratio=1.0 Adjusting this value to greater than 1.0 enables scheduling instances while over committing disk resources on the node. This might be desirable if you use an image format that is sparse or copy on write such that each virtual instance does not require a 1:1 allocation of virtual disk to physical storage.

The GroupAffinityFilter ensures that an instance is scheduled on to a host from a set of group hosts. To take advantage of this filter, the requester must pass a scheduler hint, using group as the key and an arbitrary name as the value. Using the nova command-line tool, use the --hint flag. For example: $ nova boot --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \ --hint group=foo server-1

The GroupAntiAffinityFilter ensures that each instance in a group is on a different host. To take advantage of this filter, the requester must pass a scheduler hint, using group as the key and an arbitrary name as the value. Using the nova command-line tool, use the --hint flag. For example: $ nova boot --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \ --hint group=foo server-1

Filters hosts based on properties defined on the instance's image. It passes hosts that can support the specified image properties contained in the instance. Properties include the architecture, hypervisor type, and virtual machine mode. for example, an instance might require a host that runs an ARM-based processor and QEMU as the hypervisor. An image can be decorated with these properties by using: $ glance image-update img-uuid --property architecture=arm --property hypervisor_type=qemu The image properties that the filter checks for are: architecture: Architecture describes the machine architecture required by the image. Examples are i686, x86_64, arm, and ppc64. hypervisor_type: Hypervisor type describes the hypervisor required by the image. Examples are xen, kvm, qemu, and xenapi. vm_mode: Virtual machine mode describes the hypervisor application binary interface (ABI) required by the image. Examples are 'xen' for Xen 3.0 paravirtual ABI, 'hvm' for native ABI, 'uml' for User Mode Linux paravirtual ABI, exe for container virt executable ABI.

Allows the admin to define a special (isolated) set of images and a special (isolated) set of hosts, such that the isolated images can only run on the isolated hosts, and the isolated hosts can only run isolated images. The flag restrict_isolated_hosts_to_isolated_images can be used to force isolated hosts to only run isolated images. The admin must specify the isolated set of images and hosts in the nova.conf file using the isolated_hosts and isolated_images configuration options. For example: isolated_hosts=server1,server2 isolated_images=342b492c-128f-4a42-8d3a-c5088cf27d13,ebd267a6-ca86-4d6c-9a0e-bd132d6b7d09

The JsonFilter allows a user to construct a custom filter by passing a scheduler hint in JSON format. The following operators are supported: = < > in <= >= not or and The filter supports the following variables: $free_ram_mb $free_disk_mb $total_usable_ram_mb $vcpus_total $vcpus_used Using the nova command-line tool, use the --hint flag: $ nova boot --image 827d564a-e636-4fc4-a376-d36f7ebe1747 \ --flavor 1 --hint query='[">=","$free_ram_mb",1024]' server1 With the API, use the os:scheduler_hints key:

Only schedule instances on hosts that have sufficient RAM available. If this filter is not set, the scheduler may over provision a host based on RAM (for example, the RAM allocated by virtual machine instances may exceed the physical RAM). This filter can be configured to allow a fixed amount of RAM overcommitment by using the ram_allocation_ratio configuration option in nova.conf. The default setting is: ram_allocation_ratio=1.5 This setting enables 1.5 GB instances to run on any compute node with 1 GB of free RAM.

Filter out hosts that have already been attempted for scheduling purposes. If the scheduler selects a host to respond to a service request, and the host fails to respond to the request, this filter prevents the scheduler from retrying that host for the service request. This filter is only useful if the scheduler_max_attempts configuration option is set to a value greater than zero.

Schedule the instance on the same host as another instance in a set of instances. To take advantage of this filter, the requester must pass a scheduler hint, using same_host as the key and a list of instance uuids as the value. This filter is the opposite of the DifferentHostFilter. Using the nova command-line tool, use the --hint flag: $ nova boot --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \ --hint same_host=a0cf03a5-d921-4877-bb5c-86d26cf818e1 \ --hint same_host=8c19174f-4220-44f0-824a-cd1eeef10287 server-1 With the API, use the os:scheduler_hints key:

Schedule the instance based on host IP subnet range. To take advantage of this filter, the requester must specify a range of valid IP address in CIDR format, by passing two scheduler hints: build_near_host_ip The first IP address in the subnet (for example, 192.168.1.1) cidr The CIDR that corresponds to the subnet (for example, /24) Using the nova command-line tool, use the --hint flag. For example, to specify the IP subnet 192.168.1.1/24 $ nova boot --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \ --hint build_near_host_ip=192.168.1.1 --hint cidr=/24 server-1 With the API, use the os:scheduler_hints key:

The Filter Scheduler weighs hosts based on the config option scheduler_weight_classes, this defaults to nova.scheduler.weights.all_weighers, which selects the only weigher available -- the RamWeigher. Hosts are then weighed and sorted with the largest weight winning. scheduler_weight_classes=nova.scheduler.weights.all_weighers ram_weight_multiplier=1.0 The default is to spread instances across all hosts evenly. Set the ram_weight_multiplier option to a negative number if you prefer stacking instead of spreading.

As an administrator, you work with the Filter Scheduler. However, the Compute service also uses the Chance Scheduler, nova.scheduler.chance.ChanceScheduler, which randomly selects from lists of filtered hosts.

To customize the Compute scheduler, use the configuration option settings documented in .