blockspan

Blockspan is a term used in data storage and distributed systems to refer to a contiguous range of blocks or data units that are treated as a single allocation or I/O unit. It is akin to an extent in file systems or a span in memory allocators, representing a start address or block number and a length. The concept emphasizes contiguity to improve data locality and reduce metadata overhead.

In file systems and databases, blockspan helps optimize sequential reads and writes, as the system can prefetch an entire span and minimize seeks. In distributed or cloud storage, blockspans may be transferred as a single unit, enabling efficient replication and streaming. In memory management, a blockspan can describe a chunk of pages allocated together to satisfy a allocation request.

Design and data structures: Implementations typically track spans using interval trees or balanced search trees, allowing fast allocation, deallocation, and coalescing of adjacent spans. A span might store: starting block, length in blocks, access policy, and ownership metadata. Allocation strategies may be best-fit or first-fit, with coalescing rules to minimize fragmentation. Deallocation must ensure correctness across concurrent threads or nodes in a cluster.

Challenges and considerations: Fragmentation can degrade performance; inconsistent span metadata risks data loss. Cross-block dependencies, alignment constraints, and quota enforcement must be managed. In practice, blockspan abstractions must interface with lower-level block devices, file-system inodes, or database pages.

See also: extent, block, allocation unit, inode, page, span.