Bug 1946944
| Summary: | Document machine type requirements for live migration in mixed RHEL setups | ||
|---|---|---|---|
| Product: | Red Hat OpenStack | Reporter: | Kashyap Chamarthy <kchamart> |
| Component: | documentation | Assignee: | Irina <igallagh> |
| Status: | CLOSED CURRENTRELEASE | QA Contact: | RHOS Documentation Team <rhos-docs> |
| Severity: | medium | Docs Contact: | |
| Priority: | medium | ||
| Version: | 16.1 (Train) | CC: | igallagh, lyarwood |
| Target Milestone: | --- | Keywords: | Triaged |
| Target Release: | --- | ||
| Hardware: | Unspecified | ||
| OS: | Unspecified | ||
| Whiteboard: | |||
| Fixed In Version: | Doc Type: | If docs needed, set a value | |
| Doc Text: | Story Points: | --- | |
| Clone Of: | Environment: | ||
| Last Closed: | 2022-02-02 16:26:21 UTC | Type: | Bug |
| Regression: | --- | Mount Type: | --- |
| Documentation: | --- | CRM: | |
| Verified Versions: | Category: | --- | |
| oVirt Team: | --- | RHEL 7.3 requirements from Atomic Host: | |
| Cloudforms Team: | --- | Target Upstream Version: | |
| Embargoed: | |||
Updated `NovaHWMachineType` parameter description available on the Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1/html/overcloud_parameters/compute-nova-parameters https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.2/html/overcloud_parameters/compute-nova-parameters |
Summary: Assume an OSP-17 environment based on RHEL-9, and there are two mixed RHEL compute nodes, running RHEL-9.1 and RHEL-9.2. Now, if you want to live-migrate an instance in *both* directions — i.e. from RHEL-9.1 to RHEL-9.2 and vice-versa — then you need to "pin" the machine type version that matches the older RHEL release (i.e. RHEL-9.1). * * * Here's some draft text from an old upstream document[1] that applies to OSP-16; but needs to modified for OSP-17, and be included in official docs: Live migration and versioned machine types ----------------------------------------- Today, TripleO explicitly specifies[1] versioned machine types -- to ensure migration compatibility during upgrade windows. E.g. to quote TripleO's config for machine types: ... NovaHWMachineType: description: > To specify a default machine type per host architecture. default: 'x86_64=pc-i440fx-rhel7.6.0,aarch64=virt-rhel7.6.0,ppc64=pseries-rhel7.6.0,ppc64le=pseries-rhel7.6.0' type: string ... Why? ---- Let's take a real-world example to make the point. (I'm using the Red Hat-based OpenStack, RHOS, as an example for ease of explanation. I'm sticking with "RHEL" here, otherwise the terminology gets muddied — the versioned machine types reported by CentOS's QEMU binary have 'rhel' in their names, because CentOS ships RHEL's QEMU binary as-is, minus the branding. You could replace "RHEL" with "CentOS", and the following example would be true.) Back to the example: - Assume that a RHOS deployment is running on a RHEL 7.6 host, the Nova instances will be given a RHEL 7.6 (pc-i440fx-rhel7.6.0) machine type. - And if the same deployment gets additional compute nodes added in the future, which use a newer RHEL (e.g. RHEL-7.7), the guests launched on those new compute nodes will get a newer machine type (i.e. 'pc-i440fx-rhel7.7.0'). Here's is the problem: It will now be _impossible_ to migrate a guest from a RHEL 7.7-based compute node to a RHEL 7.6-based — because RHEL 7.6 won't know about the RHEL 7.7's machine type. To deal with this trouble, you need to be able to force set an explicit machine type across all compute nodes in a deployment (as shown above). The machine type should match the latest machine type present at the time each RHOS version is released. I.e. since RHOS 14 is released initially against RHEL-7.6 base OS, Nova is configured to always use the 'rhel-7.6.0' machine type regardless of whether the compute node has upgrade to RHEL 7.7 or 7.8. This is done in ``nova.conf`` by setting the ``[libvirt]hw_machine_type``. There needs to be one machine type listed per architecture, to cover all supported. architectures on KVM. =============================================================================== [1] https://kashyapc.fedorapeople.org/versioned-machine-types-and-live-migration-gotcha.txt