Ceph: Distributed Object Storage

Ceph is a distributed, software-defined storage solution designed for scalability, reliability, and performance. It provides object storage, block storage, and file system interfaces from a single unified platform. This makes it suitable for a wide range of workloads, from cloud infrastructure and backups to archiving and high-performance computing.

Key Concepts:

• RADOS (Reliable Autonomic Distributed Object Store): The foundation of Ceph. RADOS provides the core storage capabilities, ensuring data is stored reliably and distributed across the cluster. It is responsible for data replication, self-healing, and automatic rebalancing.

• Objects: Data in Ceph is stored as objects. Each object has an ID, binary data, and metadata. Object size is configurable and is typically larger than file system blocks (e.g., 4MB). Objects are stored within Placement Groups (PGs).

• Placement Groups (PGs): Collections of objects that are managed together and are the unit of data distribution and replication. PGs determine which OSDs will store the object data. Distributing objects among PGs helps to spread the load across the cluster and facilitates parallel operations.

• OSDs (Object Storage Daemons): The workhorses of the Ceph cluster. OSDs are responsible for storing data on physical disks. Each OSD manages a set of PGs. Data is replicated across multiple OSDs for redundancy. OSDs handle data replication, recovery, and rebalancing.

• Monitors (MONs): Maintain the cluster map, which contains information about the location of all objects, OSDs, and other components. Monitors form a distributed consensus to ensure consistency of the cluster map. Ceph uses the Paxos algorithm (or a variant) to achieve consensus. An odd number of monitors (e.g., 3 or 5) is typically used for fault tolerance.

• Managers (MGRs): Provide an interface for managing and monitoring the Ceph cluster. Managers run various modules that provide functionalities such as the Ceph Dashboard, Prometheus integration, and other management tools. Unlike Monitors, Manager failures do not affect data I/O.

• CRUSH (Controlled Replication Under Scalable Hashing): The algorithm Ceph uses to determine how data is placed and replicated across the cluster OSDs. CRUSH takes into account the hardware topology of the cluster to optimize data placement for performance and resilience. CRUSH eliminates metadata bottlenecks, allowing Ceph to scale linearly. CRUSH maps allow specifying failure domains (e.g., hosts, racks, rooms) to guide replication.

Ceph Interfaces:

Ceph provides several interfaces for accessing its storage:

• Object Storage (S3/Swift): Compatible with Amazon S3 and OpenStack Swift APIs. Ideal for storing unstructured data like images, videos, and backups. Uses RADOS Gateway (RGW) to provide this. RGW allows applications designed for S3 or Swift to store data in Ceph without modification.

• Block Storage (RBD - RADOS Block Device): Provides block devices that can be attached to virtual machines or physical servers. Integrated with virtualization platforms like OpenStack and Kubernetes. Offers features like snapshots, cloning, and thin provisioning. RBD allows VMs to boot directly from Ceph storage.

• File System (CephFS): A distributed POSIX-compliant file system. Allows multiple clients to simultaneously access and modify files. Metadata is managed by Metadata Servers (MDS). CephFS supports features like snapshots, quotas, and encryption. Several active MDS daemons can improve performance and scalability.

Architecture:

Ceph's architecture is distributed and object-based:

1. Clients access data through one of the interfaces (Object, Block, or File).
2. Data is broken down into Objects.
3. Objects are placed into Placement Groups (PGs).
4. CRUSH maps PGs to OSDs (Object Storage Daemons) across the cluster.
5. OSDs store the actual object data on disk.
6. Monitors (MONs) maintain the cluster map.
7. Managers (MGRs) provide management and monitoring capabilities.

Key Features and Benefits:

• Scalability: Ceph scales horizontally by adding more OSDs to the cluster. There are theoretical limits, but production clusters of petabytes and exabytes are common.
• Reliability: Ceph replicates data across multiple OSDs to ensure data durability and availability in case of OSD failures. Erasure coding further enhances data durability while reducing storage overhead.
• High Performance: CRUSH algorithm optimizes data placement for performance and resilience. Ceph can deliver high throughput and low latency through parallel data access.
• Unified Storage: Ceph provides object, block, and file storage from a single platform, simplifying storage management.
• Self-Healing: Ceph automatically detects and recovers from OSD failures, ensuring data remains available. Rebalancing occurs automatically when capacity is added or removed.
• Software-Defined: Ceph is a software-defined storage solution, allowing it to run on commodity hardware.
• Cost-Effective: Runs on commodity hardware, reducing capital expenditure (CAPEX). Automated management reduces operational expenditure (OPEX).
• Flexibility: Ceph supports a variety of workloads, from cloud infrastructure to archiving.
• Open Source: Ceph is an open-source project, providing transparency and community support.
• Data Placement Policies: CRUSH allows for highly customizable data placement policies, allowing for optimization of data residency, performance, and cost.

Use Cases:

• Cloud Infrastructure: Storage backend for OpenStack, Kubernetes, and other cloud platforms. Providing persistent storage for virtual machines, containers and cloud native applications.
• Backup and Archiving: Storing large volumes of backup and archival data. Ceph object storage provides a cost-effective and durable solution for long-term data retention.
• Big Data Analytics: Storing and processing large datasets for big data analytics applications. Ceph's scalability and performance make it suitable for applications like Hadoop and Spark.
• Content Delivery Networks (CDNs): Storing and delivering content for CDNs. Ceph object storage can handle the high read traffic demands of CDNs.
• Media and Entertainment: Storing and streaming media content. Ceph's scalability and performance make it suitable for applications like video streaming and image hosting.
• Healthcare: Storing medical images and patient data. Ceph's reliability and security features meet the requirements of healthcare applications.
• Scientific Computing: Storing and processing large datasets for scientific simulations and research.

Comparison with other Storage Solutions:

Feature	Ceph	GlusterFS	Swift (OpenStack Object Storage)	AWS S3
Architecture	Distributed, Object-Based	Distributed, File-Based	Distributed, Object-Based	Distributed, Object-Based
Scalability	Highly Scalable	Scalable, but can be complex to manage	Highly Scalable	Highly Scalable
Data Durability	Data Replication or Erasure Coding	Data Replication or Erasure Coding	Data Replication	Data Replication
Unified Storage	Object, Block, File	File	Object	Object
Interface	S3/Swift, RBD, CephFS	NFS, SMB, FUSE	Swift API	S3 API
Open Source	Yes	Yes	Yes	No
Management	Complex, requires expertise	Simpler than Ceph, but can still be complex	Moderate	Managed Service (Simplified)
Use Cases	Cloud, Archiving, Big Data	File Sharing, Backup	Object Storage, Cloud	Object Storage, Cloud, Backup, Archival
Hardware	Commodity Hardware	Commodity Hardware	Commodity Hardware	AWS Infrastructure

Getting Started:

Setting up a Ceph cluster can be complex and typically involves these steps:

1. Hardware Planning: Plan your hardware requirements based on your workload and capacity needs. Consider the number of OSDs, monitors, and managers you need, as well as their storage capacity, memory, and CPU.
2. Operating System Installation: Install a supported Linux distribution on all the nodes in your cluster.
3. Ceph Installation: Install the Ceph packages on all the nodes. You typically need to configure a Ceph repository.
4. Monitor Configuration: Configure the monitors and create the initial cluster configuration.
5. OSD Creation: Create OSDs on the physical disks.
6. Manager Configuration: Configure the managers.
7. Pool Creation: Create storage pools, which are logical containers for objects.
8. CRUSH Map Configuration: Configure the CRUSH map to define the data placement policy.
9. Interface Configuration: Configure the interfaces (Object, Block, or File) you want to use.

Example Command (Create a Pool):

ceph osd pool create my_pool replicated 128

(Note: This command creates a replicated pool named "my_pool" with 128 placement groups. Replication will follow the configured CRUSH map.)

Conclusion:

Ceph is a powerful and versatile distributed storage solution that offers scalability, reliability, and high performance. It supports object, block, and file storage, making it suitable for a wide range of use cases. While complex compared to simpler storage solutions, its features and benefits make it a compelling choice for organizations needing a unified, software-defined storage platform. Its open-source nature and ability to run on commodity hardware contribute to its cost-effectiveness. When choosing a storage solution, evaluate your specific requirements, considering factors such as scalability, performance, cost, and ease of management.