Data Migration in Large Scale Heterogeneous Storage Systems with Nodes to Spare

In large scale storage systems such as data centers, the layout of data on storage disks needs to be frequently reconfigured for load balancing purposes or in the event of system failure/upgrades. This reconfiguration event, referred to as data migration, must be completed efficiently as the system tends to perform sub-optimally during such process. The data-migration problem has been studied extensively in the literature with efficient algorithms presented for homogeneous (all storage disks have similar capabilities) and heterogeneous (storage disks can have different capabilities) cases. In this paper, we investigate adding data forwarding to existing algorithms for the heterogeneous data migration problem. In data forwarding, we introduce additional storage nodes (called bypass nodes) during the migration process. Our simulations show that adding as few as 2 bypass nodes with limited capabilities can improve the performance by up to 15% and adding more bypass nodes with heterogeneous capabilities can improve the migration performance by 25%. We then present a novel algorithm that makes intrinsic use of bypass nodes and show that the algorithm can always achieve an optimal migration schedule while adding no more than α × n/3 bypass nodes where n is the numbers of disks and α is a term defined to reflect the heterogeneity factor of disks.