What Is Ceph?

April 3, 2024

Ceph is an open-source software platform that provides highly scalable object, block, and file-based storage under a unified system. It's built to run on commodity hardware, offering a highly reliable and easy-to-scale storage solution for large data operations. The system is designed to be self-healing and self-managing, aiming to minimize administration time and other costs.

History of Ceph

Ceph was developed by Sage Weil as part of his doctoral dissertation in computer science at the University of California, Santa Cruz (UCSC). The project began in 2004 under the direction of Professor Scott Brandt as part of the Storage Systems Research Center at UCSC.

The main goal behind Ceph was to design a distributed storage system that could scale to the exabyte level and beyond while maintaining high performance and reliability. Sage Weil and his team sought to address the limitations of existing storage solutions, which often struggled with scalability, were prone to bottlenecks, or required expensive proprietary hardware.

Here are some key milestones in the development and evolution of Ceph:

  • 2006. The initial Ceph prototype was publicly released, showcasing its innovative approach to distributed storage, including the use of the Reliable Autonomic Distributed Object Store (RADOS) to achieve high scalability and availability.
  • 2007. Ceph was released under the LGPL 2.1 (Lesser General Public License), inviting a broader community of developers to contribute to its development.
  • 2010. The first stable release of Ceph, named Argonaut, marked a significant milestone for the project, demonstrating its maturity and stability for production environments.
  • 2011. Inktank Storage was founded by Sage Weil to provide commercial support and services for Ceph, helping to accelerate its adoption in enterprise environments.
  • 2014. Red Hat, Inc. acquired Inktank Storage, further investing in the development of Ceph and integrating it into its suite of cloud and storage solutions. This acquisition was pivotal for Ceph, as it combined Red Hat's resources and expertise with Ceph's innovative technology.
  • 2015 and beyond. Ceph continued to evolve, with regular releases adding new features, improving performance, and expanding its capabilities. The community around Ceph has grown significantly, and developers, users, and companies have contributed to its development and deployment in various industries.

Ceph Architecture

Ceph's architecture is designed for scalability, reliability, and performance, leveraging the power of distributed computing to manage vast amounts of data efficiently. The architecture is fundamentally modular, allowing for the independent scaling of different components based on workload requirements. Here's an overview of the key components of Ceph's architecture.

1. RADOS (Reliable Autonomic Distributed Object Store)

RADOS is the foundation of the Ceph architecture, providing the underlying distributed storage capability. It handles data storage, data replication, and recovery. RADOS clusters are composed of two types of daemons:

  • OSDs (Object Storage Daemons). These are responsible for storing data, handling data replication, recovery, backfilling, and rebalancing across the cluster. Each OSD daemon services a storage disk and communicates with other OSDs to ensure data is consistently replicated and distributed across the cluster.
  • MONs (Monitors). Monitors maintain a master copy of the cluster map, a detailed record of the cluster state, including OSDs, their status, and other critical metadata. Monitors ensure the cluster achieves consensus on the state of the system using the Paxos algorithm, providing a reliable and consistent view of the cluster to all clients and OSDs.

2. CRUSH Algorithm

Ceph uses the CRUSH (Controlled Replication Under Scalable Hashing) algorithm to efficiently store and retrieve data. CRUSH is an innovative approach that allows Ceph to calculate where data should be stored (or retrieved from) in the cluster without needing a central lookup table. This process enables Ceph to scale horizontally without bottlenecks or single points of failure.

3. Ceph Storage Interfaces

Ceph provides multiple storage interfaces to interact with the underlying RADOS layer, catering to different storage needs:

  • RBD (RADOS Block Device). This interface provides block storage, allowing Ceph to be used as a scalable and distributed block storage solution for virtual machines and databases.
  • CephFS (Ceph File System). A POSIX-compliant file system that uses Ceph for storage, providing a file storage interface to the Ceph cluster. It offers features like snapshots and quotas.
  • RGW (RADOS Gateway). This provides object storage capabilities, offering an interface compatible with S3 and Swift APIs. It's commonly used for web-scale object storage needs.

4. Ceph Manager Daemon (ceph-mgr)

The Ceph Manager daemon is responsible for tracking runtime metrics and the current state of the cluster. It provides essential management and monitoring capabilities, ensuring that administrators have real-time insight into the cluster's health and performance.

How Does Ceph Work?

Here's a step-by-step explanation of how Ceph operates:

1. Data Distribution

In Ceph, all data is stored as objects within a flat namespace. When a file is saved to the Ceph cluster, it is divided into fixed-size blocks, which are then wrapped in objects. These objects are the basic unit of storage in Ceph, containing the data block, metadata, and a unique identifier.

Ceph uses the CRUSH algorithm to determine how to store and retrieve these objects across the cluster. CRUSH uses the unique identifier of each object to calculate which OSDs should store the object's replicas. This process allows Ceph to manage data placement in the cluster dynamically and efficiently without relying on a centralized directory or master node.

2. Data Replication

To ensure data durability and high availability, Ceph replicates each object multiple times across different OSDs. The number of replicas is configurable (typically three) to balance between redundancy and storage efficiency.

Ceph also ensures strong consistency. When data is written or modified, the changes are replicated to all copies before the write is acknowledged to the client. This ensures that all clients see the same data, regardless of which replica they access.

3. Fault Tolerance and Self-Healing

Ceph Monitors (MONs) oversee the cluster's state, including the health of OSDs and their distribution across the cluster. MONs use the Paxos consensus algorithm to agree on the cluster's current state, ensuring consistent views across nodes.

When an OSD fails, Ceph automatically redistributes its data to other OSDs, maintaining the desired level of replication. This process is known as self-healing, and it helps ensure that the system remains available and durable in the face of hardware failures.

4. Data Access

Ceph provides several interfaces for data access, each serving different storage needs:

  • RADOS Block Device (RBD) for block storage, allowing virtual machines and databases to store data on Ceph as if it were a local block device.
  • Ceph File System (CephFS) for file storage, providing a POSIX-compliant file system to store and manage files in a hierarchical structure.
  • RADOS Gateway (RGW) for object storage, offering S3 and Swift-compatible APIs for storing and accessing data as objects.

Ceph clients interact with the storage cluster through these interfaces. They use librados, a library that implements the communication protocol with Ceph OSDs, to access data stored in the cluster.

5. Scaling

Ceph's architecture allows it to scale out to thousands of nodes and petabytes to exabytes of data. Adding more storage capacity or performance is as simple as adding more nodes to the cluster. The CRUSH algorithm enables Ceph to manage this scalability efficiently, distributing the data evenly across the cluster without any central bottlenecks.

Anastazija is an experienced content writer with knowledge and passion for cloud computing, information technology, and online security. At phoenixNAP, she focuses on answering burning questions about ensuring data robustness and security for all participants in the digital landscape.