What is Block Storage?
Block storage is a storage architecture that presents raw fixed-size chunks (blocks) to a computer, which the operating system partitions and formats before use, commonly used for virtual machine disks and database volumes.
Block storage is a storage area network (SAN) style of data storage that exposes storage volumes as raw, fixed-size blocks. Each block is addressed by its logical block address (LBA), and the consumer (usually an operating system or hypervisor) is responsible for organizing those blocks into filesystems, partitions, or database extents. Unlike file storage, which handles the filesystem and directory hierarchy on the server side, block storage provides only the raw blocks over protocols such as iSCSI (RFC 3720), Fibre Channel, or NVMe-oF.
How it works: A block storage device, often called a volume or LUN (Logical Unit Number), is carved from a larger storage array or a software-defined storage solution like Ceph or OpenStack Cinder. The initiator (client) sends SCSI commands to read or write specific blocks. The array handles redundancy, snapshots, replication, and caching transparently. Because the blocks are accessed at a low level, latency is low and throughput can be very high, especially when using NVMe over Fabrics. Common block sizes are 512 bytes, 4 KiB, or 512e (512 emulation on 4K native drives).
In the wider stack, block storage is the foundation for virtual machine disks (e.g., QEMU raw files, VMware VMDK, Hyper-V VHDX) and high-performance databases (Oracle ASM, SQL Server, PostgreSQL) that manage their own storage layout. It contrasts with object storage (which uses HTTP APIs and flexible metadata) and file storage (which provides a hierarchical namespace over NFS or SMB). Block storage is typically consumed at the hypervisor or OS level, not directly by end-user applications.
Key facts
- Block storage exposes raw, fixed-size blocks addressed by LBA, not files or objects.
- Common access protocols: iSCSI (port 3260), Fibre Channel (FC), NVMe-oF (NVMe over Fabrics).
- Used by hypervisors for virtual machine disk images (VMDK, VHDX, QCOW2).
- Databases like Oracle and SQL Server often use block storage directly for performance.
- Features like snapshots and replication are handled by the storage array, not the client OS.
How it works in practice
Related terms
References
More in Storage
Cold Storage
Cold storage is a low-cost, slow-retrieval data tier for infrequently accessed information, such as backups, archives, or compliance records, with retrieval times ranging from minutes to hours.
Erasure Coding
Erasure coding is a data protection method that splits data into fragments (shards) and adds parity, allowing reconstruction from a subset of fragments; it is commonly used in cloud object storage to tolerate failures efficiently.
Fibre Channel
Fibre Channel is a high-speed networking standard primarily used to connect storage devices in storage area networks, operating as a dedicated protocol stack separate from Ethernet.
File Storage
File storage exposes data as a hierarchical filesystem over network protocols like NFS and SMB, commonly delivered via network-attached storage (NAS) appliances for shared access.
Hot Storage
Hot storage is the default high-performance tier in object storage systems, optimized for low-latency access to frequently retrieved data.
iSCSI
iSCSI is a storage protocol that encapsulates SCSI block-level commands over TCP/IP, enabling servers to access remote disks as if they were locally attached.
NFS
NFS (Network File System) is a distributed file system protocol originally developed by Sun Microsystems that allows a client to access files over a network as if they were local. It is widely used in Unix and Linux environments and runs over IP networks.
Object Storage
Object storage is a flat data storage architecture that manages data as immutable objects, each with a unique identifier, metadata, and API access over HTTP. It is the model behind Amazon S3 and similar cloud storage services.
RAID
RAID (Redundant Array of Independent Disks) combines multiple physical disk drives into a single logical unit to improve performance, fault tolerance, or both, using techniques like striping, mirroring, and parity.
RAID 0
A data storage method that splits data across two or more disks in stripes to improve read and write performance, but provides no fault tolerance.