When creating a Virtual Machine (VM) in OpenStack, several components work together in the backend to provision the instance. Here's a step-by-step explanation of how the process flows:
1. User Request
- Interface Used: The user initiates the VM creation through the Horizon Dashboard (web UI), the OpenStack CLI (
openstack server create), or the OpenStack API. - Request Information: The request includes parameters such as:
- VM name.
- Image (the operating system or custom image to boot from).
- Flavor (the compute, memory, and storage specifications).
- Network (which network the VM will be connected to).
- Security Groups (firewall rules).
- Key Pair (for SSH access).
2. API (Nova API)
- Component: Nova (Compute service).
- Process: The request is received by the Nova API, which validates the request (checks if the requested resources like the image, flavor, and network exist). If valid, the API creates an entry in the database for the new instance.
- Nova's Role: Nova is the core service responsible for managing the lifecycle of VMs in OpenStack.
3. Scheduler
- Component: Nova Scheduler.
- Process: The Nova API sends the request to the Scheduler, which is responsible for deciding which hypervisor (compute node) the VM will run on. The scheduler looks at various factors:
- Available resources (CPU, memory, disk).
- Current load on each compute node.
- Filters (such as host aggregates, affinity rules).
- Outcome: The scheduler selects the optimal compute node (hypervisor) based on the requested flavor and availability of resources.
4. Networking Setup (Neutron)
- Component: Neutron (Networking service).
- Process: Once the compute node is selected, the Nova scheduler communicates with Neutron to assign an IP address and configure network interfaces for the VM.
- DHCP: Neutron assigns the VM an IP address from the selected network's subnet, and configures the associated router to route traffic.
- Security Groups: Neutron applies the security group rules (e.g., firewall rules) to control the traffic to and from the VM.
- Networking Agents: Neutron agents (such as DHCP and L3 agents) configure networking on the compute node where the VM will run.
5. Image Service (Glance)
- Component: Glance (Image service).
- Process: Glance is responsible for providing the image (the operating system) to boot the VM.
- Image Retrieval: Nova communicates with Glance to retrieve the specified image (either from a file-based backend or from object storage like Swift).
- Image Transfer: The image is copied to the compute node, where the VM will be launched. If the image already exists in the node’s cache, it is used directly to save time.
6. Block Storage (Cinder) [Optional]
- Component: Cinder (Block Storage service).
- Process: If the user has specified an additional volume (block storage) to be attached to the VM, Nova communicates with Cinder to create the volume.
- Volume Creation: Cinder provisions a new block storage volume based on the requested size.
- Volume Attachment: Once created, the volume is attached to the VM, making it accessible as a storage device inside the VM.
7. VM Launch (Hypervisor)
- Component: Nova Compute (on the selected compute node).
- Process: Once the compute node is determined, the Nova Compute service on the selected node takes over.
- KVM/Libvirt (Hypervisor): The Nova Compute service interacts with the hypervisor (typically KVM managed by libvirt) to actually create the VM. It tells the hypervisor to:
- Allocate the necessary resources (CPU, memory, storage).
- Configure the VM’s network interfaces.
- Boot the VM from the image or volume specified.
- KVM/Libvirt (Hypervisor): The Nova Compute service interacts with the hypervisor (typically KVM managed by libvirt) to actually create the VM. It tells the hypervisor to:
- Instance Running: The VM is launched, and the user’s requested operating system starts running on the compute node.
8. Post-Launch Tasks
- Metadata Service: After the VM is booted, OpenStack injects user-specified metadata (like SSH keys or user data for cloud-init scripts) through the Metadata Service.
- SSH Key Injection: If an SSH key was specified during VM creation, the key is injected into the VM, allowing the user to log in securely.
- User Data: Cloud-init scripts (such as configuring the hostname or installing packages) are executed inside the VM based on the user’s input.
9. VM Availability
- Once all steps are completed, the VM is fully functional and accessible.
- If a floating IP was requested, the IP is assigned to the VM, making it accessible from the external network.
- The user can log into the VM via SSH, access it through the Horizon console, or use it as part of a cloud application.
Backend Data Flow: Step Summary
- API Request: User sends VM creation request via CLI, Dashboard, or API.
- Nova API: Validates request and creates a database entry.
- Scheduler: Decides the compute node for the VM based on available resources.
- Networking (Neutron): Sets up network configuration (assign IP, configure router).
- Image Service (Glance): Provides the requested image to boot the VM.
- Storage (Cinder): Creates and attaches additional block storage (if requested).
- Hypervisor (KVM/Libvirt): Boots the VM on the selected compute node.
- Metadata Injection: Adds user metadata (SSH keys, user data).
- VM Active: The VM is now running and available for the user.
Conclusion
This is a high-level overview of how VM creation works in OpenStack, showcasing how different services (Nova, Neutron, Glance, Cinder) come together to provision and configure virtual machines. Each step involves communication between various OpenStack components, ensuring the VM is created with the requested resources and network configuration.
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