redirectonwrite

Redirectonwrite, commonly written as redirect-on-write (ROW), is a data storage technique in which a write operation does not modify the existing data in place but instead redirects the write to a new location. The storage system then updates its metadata to reference the new location, making the old data obsolete or stale. This approach is often used in copy-on-write environments to ensure atomic updates and to support features such as snapshots and rollback.

How it works: when data is to be written, the system allocates fresh storage blocks, writes the new data to those blocks, and updates pointers in the metadata to reference the new location. Reads are redirected to the current data location via the metadata. The old blocks remain in storage until they are reclaimed by a background process, such as garbage collection or snapshot deletion. The technique helps maintain data integrity in the event of a crash, since partial writes do not corrupt existing data.

Relation to other concepts: ROW is commonly associated with copy-on-write file systems and storage engines. File systems that employ copy-on-write semantics, such as certain implementations found in Btrfs and ZFS, utilize ROW-like updates to achieve atomic metadata changes and robust crash recovery. In addition, some database storage layers and flash memory controllers implement ROW to minimize in-place overwrites and to enable efficient versioning and snapshotting.

Advantages and limitations: ROW provides atomicity, crash resilience, and easier implementation of snapshots and rollbacks. However, it incurs metadata overhead, potential write amplification, and can lead to fragmentation if not paired with effective garbage collection and space reclamation strategies. Performance characteristics depend on workload, metadata efficiency, and the effectiveness of the reclaim process. See also copy-on-write and related file system or storage technologies.