Host aggregates
Host aggregates are a mechanism for partitioning hosts in an
OpenStack cloud, or a region of an OpenStack cloud, based on arbitrary
characteristics. Examples where an administrator may want to do this
include where a group of hosts have additional hardware or performance
characteristics.
Host aggregates started out as a way to use Xen hypervisor resource
pools, but have been generalized to provide a mechanism to allow
administrators to assign key-value pairs to groups of machines. Each
node can have multiple aggregates, each aggregate can have multiple
key-value pairs, and the same key-value pair can be assigned to multiple
aggregates. This information can be used in the scheduler to enable
advanced scheduling, to set up Xen hypervisor resource pools or to
define logical groups for migration.
Host aggregates are not explicitly exposed to users. Instead
administrators map flavors to host aggregates. Administrators do this by
setting metadata on a host aggregate, and matching flavor extra
specifications. The scheduler then endeavors to match user requests for
instances of the given flavor to a host aggregate with the same
key-value pair in its metadata. Compute nodes can be in more than one
host aggregate. Weight multipliers can be controlled on a per-aggregate
basis by setting the desired xxx_weight_multiplier
aggregate metadata.
Administrators are able to optionally expose a host aggregate as an
Availability Zone.
Availability zones are different from host aggregates in that they are
explicitly exposed to the user, and hosts can only be in a single
availability zone. Administrators can configure a default availability
zone where instances will be scheduled when the user fails to specify
one. For more information on how to do this, refer to /admin/availability-zones.
Configure scheduler to support host
aggregates
One common use case for host aggregates is when you want to support
scheduling instances to a subset of compute hosts because they have a
specific capability. For example, you may want to allow users to request
compute hosts that have SSD drives if they need access to faster disk
I/O, or access to compute hosts that have GPU cards to take advantage of
GPU-accelerated code.
To configure the scheduler to support host aggregates, the filter_scheduler.enabled_filters
configuration option must contain the
AggregateInstanceExtraSpecsFilter in addition to the other
filters used by the scheduler. Add the following line to
nova.conf on the host that runs the
nova-scheduler service to enable host aggregates filtering,
as well as the other filters that are typically enabled:
[filter_scheduler]
enabled_filters=...,AggregateInstanceExtraSpecsFilterExample: Specify
compute hosts with SSDs
This example configures the Compute service to enable users to
request nodes that have solid-state drives (SSDs). You create a
fast-io host aggregate in the nova
availability zone and you add the ssd=true key-value pair
to the aggregate. Then, you add the node1, and
node2 compute nodes to it.
$ openstack aggregate create --zone nova fast-io
+-------------------+----------------------------+
| Field | Value |
+-------------------+----------------------------+
| availability_zone | nova |
| created_at | 2016-12-22T07:31:13.013466 |
| deleted | False |
| deleted_at | None |
| id | 1 |
| name | fast-io |
| updated_at | None |
+-------------------+----------------------------+
$ openstack aggregate set --property ssd=true 1
+-------------------+----------------------------+
| Field | Value |
+-------------------+----------------------------+
| availability_zone | nova |
| created_at | 2016-12-22T07:31:13.000000 |
| deleted | False |
| deleted_at | None |
| hosts | [] |
| id | 1 |
| name | fast-io |
| properties | ssd='true' |
| updated_at | None |
+-------------------+----------------------------+
$ openstack aggregate add host 1 node1
+-------------------+--------------------------------------------------+
| Field | Value |
+-------------------+--------------------------------------------------+
| availability_zone | nova |
| created_at | 2016-12-22T07:31:13.000000 |
| deleted | False |
| deleted_at | None |
| hosts | [u'node1'] |
| id | 1 |
| metadata | {u'ssd': u'true', u'availability_zone': u'nova'} |
| name | fast-io |
| updated_at | None |
+-------------------+--------------------------------------------------+
$ openstack aggregate add host 1 node2
+-------------------+--------------------------------------------------+
| Field | Value |
+-------------------+--------------------------------------------------+
| availability_zone | nova |
| created_at | 2016-12-22T07:31:13.000000 |
| deleted | False |
| deleted_at | None |
| hosts | [u'node1', u'node2'] |
| id | 1 |
| metadata | {u'ssd': u'true', u'availability_zone': u'nova'} |
| name | fast-io |
| updated_at | None |
+-------------------+--------------------------------------------------+Use the openstack flavor create command to create the
ssd.large flavor called with an ID of 6, 8 GB of RAM, 80 GB
root disk, and 4 vCPUs.
$ openstack flavor create --id 6 --ram 8192 --disk 80 --vcpus 4 ssd.large
+----------------------------+-----------+
| Field | Value |
+----------------------------+-----------+
| OS-FLV-DISABLED:disabled | False |
| OS-FLV-EXT-DATA:ephemeral | 0 |
| disk | 80 |
| id | 6 |
| name | ssd.large |
| os-flavor-access:is_public | True |
| ram | 8192 |
| rxtx_factor | 1.0 |
| swap | |
| vcpus | 4 |
+----------------------------+-----------+Once the flavor is created, specify one or more key-value pairs that
match the key-value pairs on the host aggregates with scope
aggregate_instance_extra_specs. In this case, that is the
aggregate_instance_extra_specs:ssd=true key-value pair.
Setting a key-value pair on a flavor is done using the openstack flavor set
command.
$ openstack flavor set \
--property aggregate_instance_extra_specs:ssd=true ssd.largeOnce it is set, you should see the extra_specs property
of the ssd.large flavor populated with a key of
ssd and a corresponding value of true.
$ openstack flavor show ssd.large
+----------------------------+-------------------------------------------+
| Field | Value |
+----------------------------+-------------------------------------------+
| OS-FLV-DISABLED:disabled | False |
| OS-FLV-EXT-DATA:ephemeral | 0 |
| disk | 80 |
| id | 6 |
| name | ssd.large |
| os-flavor-access:is_public | True |
| properties | aggregate_instance_extra_specs:ssd='true' |
| ram | 8192 |
| rxtx_factor | 1.0 |
| swap | |
| vcpus | 4 |
+----------------------------+-------------------------------------------+Now, when a user requests an instance with the ssd.large
flavor, the scheduler only considers hosts with the
ssd=true key-value pair. In this example, these are
node1 and node2.
Aggregates in Placement
Aggregates also exist in placement and are not the same thing as host
aggregates in nova. These aggregates are defined (purely) as groupings
of related resource providers. Since compute nodes in nova are
represented in placement as resource providers, they can be added to a
placement aggregate as well. For example, get the UUID of the compute
node using openstack and add it to an aggregate in placement using
hypervisor list
openstack resource provider aggregate set.
$ openstack --os-compute-api-version=2.53 hypervisor list
+--------------------------------------+---------------------+-----------------+-----------------+-------+
| ID | Hypervisor Hostname | Hypervisor Type | Host IP | State |
+--------------------------------------+---------------------+-----------------+-----------------+-------+
| 815a5634-86fb-4e1e-8824-8a631fee3e06 | node1 | QEMU | 192.168.1.123 | up |
+--------------------------------------+---------------------+-----------------+-----------------+-------+
$ openstack --os-placement-api-version=1.2 resource provider aggregate set \
--aggregate df4c74f3-d2c4-4991-b461-f1a678e1d161 \
815a5634-86fb-4e1e-8824-8a631fee3e06Some scheduling filter operations can be performed by placement for
increased speed and efficiency.
Note
The nova-api service attempts (as of nova 18.0.0) to automatically
mirror the association of a compute host with an aggregate when an
administrator adds or removes a host to/from a nova host aggregate. This
should alleviate the need to manually create those association records
in the placement API using the
openstack resource provider aggregate set CLI
invocation.
Tenant Isolation with
Placement
In order to use placement to isolate tenants, there must be placement
aggregates that match the membership and UUID of nova host aggregates
that you want to use for isolation. The same key pattern in aggregate
metadata used by the AggregateMultiTenancyIsolation filter controls this
function, and is enabled by setting scheduler.limit_tenants_to_placement_aggregate
to True.
$ openstack --os-compute-api-version=2.53 aggregate create myagg
+-------------------+--------------------------------------+
| Field | Value |
+-------------------+--------------------------------------+
| availability_zone | None |
| created_at | 2018-03-29T16:22:23.175884 |
| deleted | False |
| deleted_at | None |
| id | 4 |
| name | myagg |
| updated_at | None |
| uuid | 019e2189-31b3-49e1-aff2-b220ebd91c24 |
+-------------------+--------------------------------------+
$ openstack --os-compute-api-version=2.53 aggregate add host myagg node1
+-------------------+--------------------------------------+
| Field | Value |
+-------------------+--------------------------------------+
| availability_zone | None |
| created_at | 2018-03-29T16:22:23.175884 |
| deleted | False |
| deleted_at | None |
| hosts | [u'node1'] |
| id | 4 |
| name | myagg |
| updated_at | None |
| uuid | 019e2189-31b3-49e1-aff2-b220ebd91c24 |
+-------------------+--------------------------------------+
$ openstack project list -f value | grep 'demo'
9691591f913949818a514f95286a6b90 demo
$ openstack aggregate set \
--property filter_tenant_id=9691591f913949818a514f95286a6b90 myagg
$ openstack --os-placement-api-version=1.2 resource provider aggregate set \
--aggregate 019e2189-31b3-49e1-aff2-b220ebd91c24 \
815a5634-86fb-4e1e-8824-8a631fee3e06Note that the filter_tenant_id metadata key can be
optionally suffixed with any string for multiple tenants, such as
filter_tenant_id3=$tenantid.
Usage
Much of the configuration of host aggregates is driven from the API
or command-line clients. For example, to create a new aggregate and add
hosts to it using the openstack client, run:
$ openstack aggregate create my-aggregate
$ openstack aggregate add host my-aggregate my-hostTo list all aggregates and show information about a specific
aggregate, run:
$ openstack aggregate list
$ openstack aggregate show my-aggregateTo set and unset a property on the aggregate, run:
$ openstack aggregate set --property pinned=true my-aggregrate
$ openstack aggregate unset --property pinned my-aggregateTo rename the aggregate, run:
$ openstack aggregate set --name my-awesome-aggregate my-aggregateTo remove a host from an aggregate and delete the aggregate, run:
$ openstack aggregate remove host my-aggregate my-host
$ openstack aggregate delete my-aggregateFor more information, refer to the OpenStack.
Client documentation <cli/command-objects/aggregate.html>
Configuration
In addition to CRUD operations enabled by the API and clients, the
following configuration options can be used to configure how host
aggregates and the related availability zones feature operate under the
hood:
default_schedule_zonescheduler.limit_tenants_to_placement_aggregatecinder.cross_az_attach
Finally, as discussed previously, there are a number of host
aggregate-specific scheduler filters. These are:
AggregateImagePropertiesIsolationAggregateInstanceExtraSpecsFilterAggregateIoOpsFilterAggregateMultiTenancyIsolationAggregateNumInstancesFilterAggregateTypeAffinityFilter
The following configuration options are applicable to the scheduler
configuration:
cpu_allocation_ratioram_allocation_ratiofilter_scheduler.max_instances_per_hostfilter_scheduler.aggregate_image_properties_isolation_separatorfilter_scheduler.aggregate_image_properties_isolation_namespace
Image Caching
Aggregates can be used as a way to target multiple compute nodes for
the purpose of requesting that images be pre-cached for performance
reasons.
Note
Some
of the virt drivers provide image caching support, which improves
performance of second-and-later boots of the same image by keeping the
base image in an on-disk cache. This avoids the need to re-download the
image from Glance, which reduces network utilization and time-to-boot
latency. Image pre-caching is the act of priming that cache with images
ahead of time to improve performance of the first boot.
Assuming an aggregate called my-aggregate where two
images should be pre-cached, running the following command will initiate
the request:
$ nova aggregate-cache-images my-aggregate image1 image2Note that image pre-caching happens asynchronously in a best-effort
manner. The images and aggregate provided are checked by the server when
the command is run, but the compute nodes are not checked to see if they
support image caching until the process runs. Progress and results are
logged by each compute, and the process sends
aggregate.cache_images.start,
aggregate.cache_images.progress, and
aggregate.cache_images.end notifications, which may be
useful for monitoring the operation externally.
