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Data storage systems and their availability play a crucial role incontemporary datacenters. Despite using mechanisms such as automatic fail-overin datacenters, the role of human agents and consequently their destructiveerrors is inevitable. Due to very large number of disk drives used in exascaledatacenters and their high failure rates, the disk subsystem in storage systemshas become a major source of Data Unavailability (DU) and Data Loss (DL)initiated by human errors. In this paper, we investigate the effect ofIncorrect Disk Replacement Service (IDRS) on the availability and reliabilityof data storage systems. To this end, we analyze the consequences of IDRS in adisk array, and conduct Monte Carlo simulations to evaluate DU and DL duringmission time. The proposed modeling framework can cope with a) differentstorage array configurations and b) Data Object Survivability (DOS),representing the effect of system level redundancies such as remote backups andmirrors. In the proposed framework, the model parameters are obtained fromindustrial and scientific reports alongside field data which have beenextracted from a datacenter operating with 70 storage racks. The results showthat ignoring the impact of IDRS leads to unavailability underestimation by upto three orders of magnitude. Moreover, our study suggests that by consideringthe effect of human errors, the conventional beliefs about the dependability ofdifferent Redundant Array of Independent Disks (RAID) mechanisms should berevised. The results show that RAID1 can result in lower availability comparedto RAID5 in the presence of human errors. The results also show that employingautomatic fail-over policy (using hot spare disks) can reduce the drasticimpacts of human errors by two orders of magnitude.

Modeling Impact of Human Errors on the Data Unavailability and Data Loss of Storage Systems