Add "RBD" section to architecture.md

docs/architecture/ceph.md

@@ -13,6 +13,130 @@ Storage Cluster accommodates large numbers of nodes, which communicate with
 each other to replicate and redistribute data dynamically.
 
 ## Architecture
+
+## Ceph Block Device Summary (RBD)
+
+### Overview of RBD
+
+A block is a sequence of bytes, often 512 bytes in size. Block-based storage
+interfaces represent a mature and common method for storing data on various
+media types including hard disk drives (HDDs), solid-state drives (SSDs),
+compact discs (CDs), floppy disks, and magnetic tape. The widespread adoption
+of block device interfaces makes them an ideal fit for mass data storage
+applications, including their integration with Ceph storage systems.
+
+### Core Features
+
+Ceph block devices are designed with three fundamental characteristics:
+thin-provisioning, resizability, and data striping across multiple Object
+Storage Daemons (OSDs). These devices leverage the full capabilities of RADOS
+(Reliable Autonomic Distributed Object Store), including snapshotting,
+replication, and strong consistency guarantees. Ceph block storage clients
+establish communication with Ceph clusters through two primary methods: kernel
+modules or the librbd library.
+
+An important distinction exists between these two communication methods
+regarding caching behavior. Kernel modules have the capability to utilize Linux
+page caching for performance optimization. For applications that rely on the
+librbd library, Ceph provides its own RBD (RADOS Block Device) caching
+mechanism to enhance performance.
+
+### Performance and Scalability
+
+Ceph's block devices are engineered to deliver high performance combined with
+vast scalability capabilities. This performance extends to various deployment
+scenarios, including direct integration with kernel modules and virtualization
+environments. The architecture supports Key-Value Machines (KVMs) such as QEMU,
+enabling efficient virtualized storage operations.
+
+Cloud-based computing platforms have embraced Ceph block devices as a storage
+backend solution. Major cloud computing systems including OpenStack, OpenNebula,
+and CloudStack integrate with Ceph block devices through their reliance on
+libvirt and QEMU technologies. This integration allows these cloud platforms to
+leverage Ceph's distributed storage capabilities for their virtual machine
+storage requirements.
+
+### Unified Storage Cluster
+
+One of Ceph's significant architectural advantages is its ability to support
+multiple storage interfaces simultaneously within a single cluster. The same
+Ceph cluster can concurrently operate the Ceph RADOS Gateway for object
+storage, the Ceph File System (CephFS) for file-based storage, and Ceph block
+devices for block-based storage. This unified approach eliminates the need for
+separate storage infrastructure for different storage paradigms, simplifying
+management and reducing operational overhead.
+
+This multi-interface capability allows organizations to deploy a single storage
+solution that addresses diverse storage requirements, from traditional block
+storage for databases and virtual machines to object storage for unstructured
+data and file storage for shared filesystems. The convergence of these storage
+types within one cluster provides operational efficiency and cost-effectiveness
+while maintaining the performance and reliability characteristics required for
+enterprise deployments.
+
+### Technical Implementation
+
+The thin-provisioning feature of Ceph block devices means that storage space is
+allocated only as data is written, rather than pre-allocating the entire volume
+capacity upfront. This approach optimizes storage utilization by avoiding waste
+from unused pre-allocated space and allows for oversubscription strategies
+where the sum of provisioned capacity can exceed physical capacity, based on
+actual usage patterns.
+
+The resizable nature of Ceph block devices provides operational flexibility,
+allowing administrators to expand or contract volume sizes based on changing
+application requirements without disrupting service availability. This dynamic
+sizing capability supports evolving storage needs without requiring complex
+migration procedures or extended downtime windows.
+
+Data striping across multiple OSDs distributes data blocks across the cluster's
+storage nodes. This distribution achieves two critical objectives: it increases
+aggregate throughput by allowing parallel I/O operations across multiple
+devices, and it ensures data availability through the replication mechanisms
+built into RADOS. The striping process breaks data into smaller chunks that are
+distributed according to the cluster's CRUSH (Controlled Scalable Decentralized
+Placement of Replicated Data) algorithm, which determines optimal placement
+based on cluster topology and configured policies.
+
+### RADOS Integration
+
+The integration with RADOS provides Ceph block devices with enterprise-grade
+features. Snapshotting capability enables point-in-time copies of block devices,
+supporting backup operations, testing scenarios, and recovery procedures.
+Snapshots are space-efficient, storing only changed data rather than full
+copies, and can be created instantaneously without impacting ongoing operations.
+
+Replication ensures data durability by maintaining multiple copies of data
+across different cluster nodes. The replication factor is configurable,
+allowing organizations to balance storage efficiency against data protection
+requirements. Strong consistency guarantees ensure that all replicas reflect the
+same data state, preventing split-brain scenarios and ensuring data integrity
+even during failure conditions.
+
+The communication architecture between block storage clients and Ceph clusters
+through kernel modules or librbd provides flexibility in deployment scenarios.
+Kernel module integration enables direct access from operating systems, while
+librbd allows applications to interact with Ceph block devices programmatically,
+supporting a wide range of use cases from bare-metal servers to containerized
+applications.
+
+### Conclusion
+
+Ceph block devices represent a sophisticated implementation of block storage
+that combines the traditional simplicity of block-based interfaces with modern
+distributed storage capabilities. The thin-provisioned, resizable architecture
+with data striping across multiple OSDs provides a foundation for scalable,
+high-performance storage. Integration with RADOS brings enterprise features
+including snapshotting, replication, and strong consistency, while support for
+both kernel modules and librbd ensures broad compatibility across deployment
+scenarios. The ability to run block devices alongside object and file storage
+within a unified cluster positions Ceph as a comprehensive storage solution
+capable of addressing diverse organizational storage requirements through a
+single infrastructure platform. This convergence of capabilities, combined with
+proven integration with major virtualization and cloud platforms, establishes
+Ceph block devices as a viable solution for modern data center storage needs.
+
+## See Also
 The architecture of the Ceph cluster is explained in [the Architecture
 chapter of the upstream Ceph
 documentation](https://docs.ceph.com/en/latest/architecture/)