Replication-based distributed storage systems with variable repetition degrees

We consider a class of regenerating codes for distributed storage systems that provide exact and uncoded repair at the minimum bandwidth regenerating point. The desirable properties can be achieved by a coding scheme which concatenates an outer MDS code and an inner fractional repetition (FR) code. However, it is desirable to have the more popular packets with higher repetition degrees in practical systems. Motivated by this, we propose a new framework of code design where the repetition degrees of coded packets can be different. By adopting group divisible designs, our framework allows the design of system over a large range of parameters. Moreover, we make use of the systematic feature of MDS codes and wisely partition the storage nodes into several clusters. We show that in normal cases, data reconstruction time can be greatly reduced while contacting nodes in the same cluster.