Cinder and Ceph are not competing technologies but rather complementary components that address different layers of the storage stack. Cinder is a block storage service and API for the OpenStack cloud platform, while Ceph is a distributed storage system that can act as a high-performance, scalable backend for Cinder.
Fundamental differences
| Feature | Cinder | Ceph |
|---|---|---|
| Role | Provides a standardized API and a set of services for managing block storage volumes within an OpenStack environment. It is not a storage solution itself. | A complete, software-defined storage solution that provides block, object, and file storage in a single, unified system. |
| Position in Stack | A higher-level abstraction layer that works with various storage backends. It sits within the OpenStack control plane, translating API requests into commands for the underlying storage. | The underlying, low-level storage infrastructure. It manages data placement, replication, and distribution across physical hardware. |
| Architecture | A collection of services (cinder-api, cinder-volume, cinder-scheduler) that manage the lifecycle of block storage volumes. It uses vendor-specific drivers to communicate with different storage systems. |
A distributed cluster of storage daemons (OSDs, MONs, MDSs) that work together to store and retrieve data. It uses the CRUSH algorithm for data distribution. |
| Storage Types | Primarily focused on block storage (volumes) for OpenStack virtual machines. | Offers unified storage, providing block storage (RBD), object storage (RGW), and file storage (CephFS). |
| Scalability | Its scalability is tied to the underlying storage backend. It can manage multiple backends but doesn't handle the data distribution itself. | Highly scalable and designed for massive, exabyte-scale clusters. It can scale out by adding more commodity hardware nodes. |
| OpenStack Integration | A native and essential component of the OpenStack ecosystem. | A common storage backend for OpenStack, integrating with Cinder (for block storage), Glance (for images), and Nova (for ephemeral disks). |
Detailed exploration
Cinder: The OpenStack block storage service
Cinder is the block storage service for OpenStack, responsible for providing persistent storage volumes to virtual machines (VMs). It provides a standardized API that abstracts the complexity of the underlying storage hardware. This abstraction allows a cloud administrator to present a consistent storage experience to users, regardless of the physical storage system being used.
Key functions of Cinder:
- Volume Lifecycle Management: Cinder handles the creation, deletion, attachment, and detachment of volumes to OpenStack instances.
- Volume Snapshots and Backups: It manages point-in-time, read-only copies of volumes (snapshots) and can also back up volumes to a dedicated repository, like an object store.
- Multi-Backend Support: Cinder can interface with numerous storage backends through a system of drivers. This allows an OpenStack environment to use different types of storage, such as local LVM, proprietary vendor solutions (NetApp, SolidFire), and distributed systems like Ceph.
- Quality of Service (QoS): Cinder can enforce performance characteristics (like IOPS) on volumes by utilizing the QoS features of the backend storage.
- Volume Types: Administrators can define volume types with specific capabilities and QoS settings. The Cinder scheduler then matches user volume requests to the appropriate backend based on these volume types.
Ceph: The unified, distributed storage system
Ceph is a powerful, open-source software-defined storage solution that runs on commodity hardware. Its core architecture, known as RADOS (Reliable Autonomic Distributed Object Store), allows it to store and manage massive amounts of data in a self-managing, self-healing, and fault-tolerant cluster.
Key features of Ceph:
- Unified Storage: Ceph provides a single storage cluster that can serve different types of storage needs simultaneously.
- Block Storage (RBD): RADOS Block Device (RBD) provides block-based storage interfaces that are thin-provisioned, resizable, and ideal for VMs and containers.
- Object Storage (RGW): The Ceph RADOS Gateway (RGW) provides a RESTful interface compatible with Amazon S3 and OpenStack Swift APIs.
- File Storage (CephFS): CephFS is a POSIX-compliant file system that provides a familiar file storage interface.
- Distributed Architecture: Data is broken into objects and distributed across the cluster based on the CRUSH (Controlled Replication Under Scalable Hashing) algorithm. CRUSH dynamically calculates the best placement for data, eliminating single points of failure and enabling massive scalability.
- High Availability and Reliability: Ceph ensures data durability through replication or erasure coding. It automatically handles data rebalancing and recovery in the event of a disk or node failure.
- Scalability: The system is designed to scale out horizontally by adding more commodity hardware. This allows for independent scaling of performance and capacity, preventing bottlenecks.
The synergy: Cinder and Ceph working together
In a common cloud infrastructure deployment, particularly with OpenStack, Cinder and Ceph are used together to provide a robust and scalable block storage solution.
How they integrate:
- Ceph as the Backend: The cloud administrator first sets up a Ceph cluster, which provides the actual storage infrastructure.
- Cinder Driver: Within the OpenStack environment, the Cinder service is configured to use the Ceph RBD driver.
- User Request: An OpenStack user requests a new block storage volume through the Cinder API.
- Cinder-Ceph Interaction: Cinder, via its Ceph driver, translates this request into a command for the Ceph cluster.
- Ceph Provisioning: Ceph's RBD service provisions the new block device within a designated storage pool on the Ceph cluster.
- Volume Attachment: Cinder then attaches this provisioned block device to the user's OpenStack instance.
This combination offers significant benefits for large-scale production deployments:
- Decoupled Storage: The storage volumes are decoupled from the compute nodes, enabling live migration of VMs and robust disaster recovery capabilities.
- High Performance and Scalability: The distributed nature of Ceph allows for massive scalability and high-performance I/O for Cinder volumes.
- Reliability: Ceph's built-in replication or erasure coding ensures data redundancy for Cinder volumes.
- Efficient Volume Operations: When using Ceph as a backend, features like snapshots and volume creation are highly efficient due to Ceph's copy-on-write functionality. New volumes can be created almost instantly from a snapshot, as only the new data needs to be written.
Conclusion
While Cinder and Ceph both relate to storage, they operate at different layers of the infrastructure stack and serve different purposes. Cinder is an application-level service that offers an abstract API for block storage management within OpenStack. In contrast, Ceph is the low-level, software-defined storage system that can provide the actual, physical storage capacity for that service. In most large-scale OpenStack deployments, the two are used in tandem, with Ceph serving as the robust and scalable backend that powers Cinder's block storage capabilities.