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The MTBF of a Hadoop node and an enterprise storage server is three and six years respectively [6], [20]. Based on these values, Table I compares the probability of data loss per day in HDFS and NAS filers using Equations 3. We compare the HDFS cluster with 1000 nodes to a filer with 100 nodes assuming that they can offer the same storage capacity. This is valid assumption given recent trends in storage capacity. Enterprise systems can easily support more than 240 TB of storage [24], while a typical Hadoop node has 12 TB to 24 TB of storage. Filers can offer probability of data loss with one replica of 4.4 ∗ 10−2 , compared to HDFS with 3 replicas, which has a 6.44 ∗ 10−2 probability of data loss. Given the high number of disks in a single enterprise storage node, fault tolerance is handled by a RAID controller with a probability of data loss of 4.4 ∗ 10−3 , and hence in this case we decrease the replication factor to 1. Disks are arranged in, for example, a (10, 2) RAID array, which protects from a simultaneous loss of two disks with ten-fold decrease in storage overhead when compared to two replicas. Thus, the use of filers as secondary storage is promising in AptStore and offer a most cost-efficient yet robust solutio