tailieunhanh - THE FRACTAL STRUCTURE OF DATA REFERENCE- P24

THE FRACTAL STRUCTURE OF DATA REFERENCE- P24:For purposes of understanding its performance, a computer system is traditionally viewed as a processor coupled to one or more disk storage devices, and driven by externally generated requests (typically called transactions). Over the past several decades, very powerful techniques have become available to the performance analyst attempting to understand, at a high level, the operational behavior of such systems. | Hierarchical Storage Management 105 level 0 to level 1 and both levels use the same type of disk hardware then the cost of level 1 storage would be one-halfthat of level 0. Although we wish to determine the amount of primary disk storage by modeling it is also desirable to ensure some minimum amount of primary storage. Even ifthe storage management policy specifies the fastest possible migration migration after 0 days some primary storage will still be needed for data currently in use for free space and as a buffer for data being migrated or recalled. The model allows this minimum storage to be specified as a fixed requirement. Our storage management model therefore ends up using the following variables - minimum primary storage gigabytes . v. primary storage beyond the minimum gigabytes . level 1 disk storage gigabytes . V So Si total disk storage beyond the minimum gigabytes . E . cost ofprimary storage per gigabyte per day . cost oflevel 1 storage after accounting for compression per gigabyte per day Ei Eo . On - recall delay due to miss in level 0 time to recall from level 1 seconds . O recall delay due to miss in level 1 time to recall from level 2 seconds Di Do . in - level 0 miss probability per i o probability that the requested data is not at level 0 . n -. --- level 1 miss probability per i o probability that the requested data is neither at level 0 nor level 1 . D target delay per i o seconds . migration age period of non-use before migrating data from level 0 to level 1 days . r i migration age period of non-use before migrating data from level 1 to level 2 days Ti To . In terms ofthese variables we wish to accomplish the following Constrained optimization version A Find the two values Sd So 0 such that EqSqo EqSo E S 106 THE FRACTAL STRUCTURE OF DATA REFERENCE is a minimum subject to Da m0 mj D m D Constrained optimization version A is not yet ready however to apply in practice. First we must quantify how the level 0 and level 1 miss ratios mo and m1 .