Simplex Queues for Hot-Data Download

In distributed systems, reliable data storage is accomplished through redundancy, which has traditionally been achieved by simple replication of data across multiple nodes [6]. A special class of erasure codes, known as locally repairable codes (LRCs) [7], has started to replace replication in practice [8], as a more storage-efficient way to provide a desired reliability. It has recently been recognized, that storage redundancy can also provide fast access of stored data (see e.g. [5,9,10] and references therein). Most of these papers consider download scenarios of all jointly encoded pieces of data, and very few [11,12,14] are concerned with download of only some, possibly hot, pieces of data that are jointly encoded with those of less interest. So far, only low traffic regime has been partially addressed. In this paper, we are concerned with hot data download from systems implementing a special class of locally repairable codes, known as LRCs with availability [13,15]. We consider simplex codes, a particular subclass of LRCs with availability, because 1) they are in a certain sense optimal [2] and 2) they are minimally different from replication.