======================= Resize and cold migrate ======================= The `resize API`_ and `cold migrate API`_ are commonly confused in nova because the internal `API code`_, `conductor code`_ and `compute code`_ use the same methods. This document explains some of the differences in what happens between a resize and cold migrate operation. For the most part this document describes :term:`same-cell resize `. For details on :term:`cross-cell resize `, refer to :doc:`/admin/configuration/cross-cell-resize`. High level ~~~~~~~~~~ :doc:`Cold migrate ` is an operation performed by an administrator to power off and move a server from one host to a **different** host using the **same** flavor. Volumes and network interfaces are disconnected from the source host and connected on the destination host. The type of file system between the hosts and image backend determine if the server files and disks have to be copied. If copy is necessary then root and ephemeral disks are copied and swap disks are re-created. :doc:`Resize ` is an operation which can be performed by a non-administrative owner of the server (the user) with a **different** flavor. The new flavor can change certain aspects of the server such as the number of CPUS, RAM and disk size. Otherwise for the most part the internal details are the same as a cold migration. Scheduling ~~~~~~~~~~ Depending on how the API is configured for :oslo.config:option:`allow_resize_to_same_host`, the server may be able to be resized on the current host. *All* compute drivers support *resizing* to the same host but *only* the vCenter driver supports *cold migrating* to the same host. Enabling resize to the same host is necessary for features such as strict affinity server groups where there are more than one server in the same affinity group. Starting with `microversion 2.56`_ an administrator can specify a destination host for the cold migrate operation. Resize does not allow specifying a destination host. Flavor ~~~~~~ As noted above, with resize the flavor *must* change and with cold migrate the flavor *will not* change. Resource claims ~~~~~~~~~~~~~~~ Both resize and cold migration perform a `resize claim`_ on the destination node. Historically the resize claim was meant as a safety check on the selected node to work around race conditions in the scheduler. Since the scheduler started `atomically claiming`_ VCPU, MEMORY_MB and DISK_GB allocations using Placement the role of the resize claim has been reduced to detecting the same conditions but for resources like PCI devices and NUMA topology which, at least as of the 20.0.0 (Train) release, are not modeled in Placement and as such are not atomic. If this claim fails, the operation can be rescheduled to an alternative host, if there are any. The number of possible alternative hosts is determined by the :oslo.config:option:`scheduler.max_attempts` configuration option. Allocations ~~~~~~~~~~~ Since the 16.0.0 (Pike) release, the scheduler uses the `placement service`_ to filter compute nodes (resource providers) based on information in the flavor and image used to build the server. Once the scheduler runs through its filters and weighers and picks a host, resource class `allocations`_ are atomically consumed in placement with the server as the consumer. During both resize and cold migrate operations, the allocations held by the server consumer against the source compute node resource provider are `moved`_ to a `migration record`_ and the scheduler will create allocations, held by the instance consumer, on the selected destination compute node resource provider. This is commonly referred to as `migration-based allocations`_ which were introduced in the 17.0.0 (Queens) release. If the operation is successful and confirmed, the source node allocations held by the migration record are `dropped`_. If the operation fails or is reverted, the source compute node resource provider allocations held by the migration record are `reverted`_ back to the instance consumer and the allocations against the destination compute node resource provider are dropped. Summary of differences ~~~~~~~~~~~~~~~~~~~~~~ .. list-table:: :header-rows: 1 * - - Resize - Cold migrate * - New flavor - Yes - No * - Authorization (default) - Admin or owner (user) Policy rule: ``os_compute_api:servers:resize`` - Admin only Policy rule: ``os_compute_api:os-migrate-server:migrate`` * - Same host - Maybe - Only vCenter * - Can specify target host - No - Yes (microversion >= 2.56) Sequence Diagrams ~~~~~~~~~~~~~~~~~ The following diagrams are current as of the 21.0.0 (Ussuri) release. Resize ------ This is the sequence of calls to get the server to ``VERIFY_RESIZE`` status. .. image:: /_static/images/resize/resize.svg :alt: Resize standard workflow Confirm resize -------------- This is the sequence of calls when confirming `or deleting`_ a server in ``VERIFY_RESIZE`` status. Note that in the below diagram, if confirming a resize while deleting a server the API synchronously calls the source compute service. .. image:: /_static/images/resize/resize_confirm.svg :alt: Resize confirm workflow Revert resize ------------- This is the sequence of calls when reverting a server in ``VERIFY_RESIZE`` status. .. image:: /_static/images/resize/resize_revert.svg :alt: Resize revert workflow .. _resize API: https://docs.openstack.org/api-ref/compute/#resize-server-resize-action .. _cold migrate API: https://docs.openstack.org/api-ref/compute/#migrate-server-migrate-action .. _API code: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/compute/api.py#L3568 .. _conductor code: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/conductor/manager.py#L297 .. _compute code: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/compute/manager.py#L4445 .. _microversion 2.56: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id52 .. _resize claim: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/compute/resource_tracker.py#L248 .. _atomically claiming: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/scheduler/filter_scheduler.py#L239 .. _moved: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/conductor/tasks/migrate.py#L28 .. _placement service: https://docs.openstack.org/placement/latest/ .. _allocations: https://docs.openstack.org/api-ref/placement/#allocations .. _migration record: https://docs.openstack.org/api-ref/compute/#migrations-os-migrations .. _migration-based allocations: https://specs.openstack.org/openstack/nova-specs/specs/queens/implemented/migration-allocations.html .. _dropped: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/compute/manager.py#L4048 .. _reverted: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/compute/manager.py#L4233 .. _or deleting: https://opendev.org/openstack/nova/src/tag/19.0.0/nova/compute/api.py#L2135