N-Code: An Optimal RAID-6 MDS Array Code for Load Balancing and High I/O Performance

Existing RAID-6 codes are developed to optimize either reads or writes for storage systems. To improve both read and write operations, this paper proposes a novel RAID-6 MDS array code called N-Code. N-Code exhibits three aspects of salient features: (i) read performance. N-Code assigns both horizontal parity chains and horizontal parities across disks, without generating a dedicated parity disk. Such a parity layout not only makes all the disks service normal reads, but also allows continuous data elements to share the same horizontal chain to optimize degraded reads; (ii) write performance. Diagonal parities are distributed across disks in a decentralized manner to optimize partial stripe writes, and horizontal parity chains enable N-Code to reduce I/O costs of partial stripe writes by merging I/O operations; and (iii) balancing performance. Decentralized horizontal/diagonal parities potentially support the I/O balancing optimization for single writes.

A theoretical analysis indicates that apart from the optimal storage efficiency, N-Code is featured with the optimal complexity for both encoding/decoding computations and update operations. The results of empirical experiments shows that N-Code demonstrates higher normal-read, degraded-read, and partial-stripe-write performance than the seven baseline popular RAID-6 codes. In particular, in the partial-stripe-write case, N-Code accelerates partial stripe writes by 32%-66% relative to horizontal codes; when it comes to degraded reads, N-Code improves degraded reads by 32%-53% compared to vertical codes. Furthermore, compared to the baseline codes, N-Code enhances load balancing by a factor anywhere between 1.19 to 9.09 for single-write workload, and between 1.3 to 6.92 for read-write mixed workload.