Availability codes have recently been proposed to facilitate efficient storage, management, and retrieval of frequently accessed data in distributed storage systems. Such codes provide multiple disjoint recovery groups for each data object, which makes it possible for multiple users to access the same object in a non-overlapping way. However in the presence of server-side performance variability, downloading an object using a recovery group takes longer than using a single server hosting the object. Therefore it is not immediately clear whether availability codes reduce latency to access hot data . Accordingly, the goal of this paper is to analyze, using a queuing theoretical approach, the download time in storage systems that employ availability codes . For data access, we consider the widely adopted Fork-Join model with redundancy. In this model, each request arrival splits into multiple copies and completes as soon as any one of the copies finishes service. We first carry out the analysis under the low-traffic regime in which case the system consists of at most one download request at any time. In this setting, we compare the download time in  systems with availability, maximum distance separable (MDS), and replication codes. Our results indicate that availability codes can reduce download time in some settings, but are not always optimal. When the low-traffic assumption does not hold, system consists of multiple inter-dependent Fork-Join queues operating in parallel , which makes the exact analysis intractable  . For this case  we present upper and lower bounds on the download time . These bounds yield insight on system performance with respect to varying popularities over the stored objects. We also derive an M/G/1 queue approximation for the system, and show with simulations that it performs well in estimating the actual system performance .
Availability codes have recently been proposed to facilitate efficient retrieval of frequently accessed (hot) data objects in distributed storage systems. Such codes provide multiple disjoint recovery groups for each data object, thus providing flexibility in accessing data objects. This paper presents techniques for analyzing the download time of systematic availability codes considering the Fork-Join scheme for data access. With the Fork-Join access scheme, each data request is replicated (forked) to the systematic server containing the object and all of its recovery groups. For low-traffic regime , when there is at most one request in the system, we compute the download time in closed-form and compare it across systems with availability, maximum distance separable (MDS), and replication codes. We demonstrate that availability codes can reduce download time in some settings, but are not always optimal. When the low-traffic assumption does not hold, system consists of multiple inter-dependent Fork-Join queues , which makes exact analysis intractable due to state space explosion . For this case , we present upper and lower bounds on the download time , and an M/G/1 queue approximation for several special cases of interest. Via numerical simulations, we evaluate our bounds, and demonstrate that the M/G/1 queue approximation has a high degree of accuracy. We also compare availability codes with MDS and replication codes .
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storage, management, and
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in storage systems that employ availability codes . For data access, we consider the widely adopted
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due to state space explosion
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. These bounds yield insight on system performance with respect to varying popularities over the stored objects. We also derive
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the system, and show with simulations that it performs well in estimating the actual system performance
several special cases of interest. Via numerical simulations, we evaluate our bounds, and demonstrate that the M/G/1 queue approximation has a high degree of accuracy. We also compare availability codes with MDS and replication codes