Where chained declustering is used?

Answer 1

Chained De-clustering is a method used to manage replicated data. It is usually implemented in a shared nothing architecture. This technique is capable of providing both high availability in addition to being able to fully balance the workload among the operational nodes in the event of a failure. In the chained declustering strategy, nodes are divided into disjoint groups called relation-clusters. The tuples of each relation in the database are declustered across the disk drives of one relation cluster. The chained declustering strategy maintains two physical copies of each relation. The first copy (termed the primary copy) is declustered over all disks in a relation-cluster using one of Gamma’s three partitioning strategies (hashed, range, or round-robin) [DeWi86]. The second copy (termed the backup copy) is declustered using the same partitioning strategy but the corresponding fragments from both copies are stored on different nodes. The fragments are distributed among the disks using the following algorithm: Assume that there are a total of M disks numbered from 1 to M. For every relation R, the i th fragment of the primary copy is stored on the {[i-1+C(R)] mod M + 1}-th disk drive, and the i th fragment of the backup copy will be stored on the {[i+C(R)] mod M + 1}-th7 disk drive; insuring that the primary and backup fragments are placed on different disk drives. The function C(R) is devised to allow the first fragment of relation R to be placed on any disk within a relation-cluster. We name this data replication method chained declustering because the disks are linked together, by the fragments of a relation, like a chain.