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ObjectivesIn this work, we present a simple solution to this problem by proposing AdaptRaid0 and AdaptRaid5, two block-distribution algorithm for disk arrays level 0 and 5 that improve the performance and effective capacity of heterogeneous disk arrays compared to current solutions. People Involved
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MotivationHeterogeneous disk arrays are becoming (or will be in a near future) a common configuration in many sites. Let us describe two scenarios that end up in a heterogeneous disk array. The first one appears whenever a component of a traditional array fails and it has to be replaced by a new one. As disk technology improves quite rapidly, it is quite probable for the new disk to be faster and larger than the ones already in the array . A similar scenario appears when the capacity needs of a site grow and new disks have to be acquired to grow the size of the array (by increasing the number of disks in the array). In this case, it will also be difficult to buy the same disks as the ones in the original configuration, and thus newer disks will be added. In both cases, we will make the array a heterogeneous one because it will be made of disks with different characteristics. This kind of situation is especially common in low-cost clusters of workstations, where cost is an important issue and old components have to be used as well as possible. According to the study performed by Dr. Grochowski at IBM, disk capacity nearly doubles every year while the price per Mbyte is decreasing about 40% per year. This means that the price of arrays will remain about the same throughout the years, although the capacity will be increased a lot, of course. To handle this kind of disk array, current systems do not take into account the differences between the disks. All disks are treated as if they had same capacity (the smallest one) and performance (the slowest one). This is not the best approach because improvements in both capacity and response time of the heterogeneous array could be achieved if each disk were used accordingly to its characteristics. Related Publications
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