Locally Repairable Regenerating Codes: Node Unavailability and the Insufficiency of Stationary Local Repair

Abstract

Recent works by Ahmad et al. and by Hollmann studied the concept of “locally repairable regenerating codes (LRRCs)” that successfully combines the functional repair and partial information exchange of regenerating codes (RCs) with the much-desired local repairability feature of locally repairable codes (LRCs). One important issue that needs to be addressed by any local repair schemes (including both LRCs and LRRCs) is that sometimes designated helper nodes may be temporarily unavailable, the result of various reasons that include multiple failures, degraded reads, or power-saving strategies to name a few. Under the setting of LRRCs with temporary node unavailability, this paper studies the impact of different helper selection methods. It proves that with node unavailability, all existing methods of helper selection, including those used in RCs and LRCs, can be insufficient in terms of achieving the optimal repair-bandwidth. For some scenarios, it is necessary to combine LRRCs with a new class of helper selection methods, termed dynamic helper selection , to achieve optimal repair-bandwidth. This paper also compares the performance of different classes of helper selection methods and answers the following fundamental question: is one method of helper selection intrinsically better than the other? for various scenarios.