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The proxy workload is characterized by a large variability in request rate and frequent idle periods. Using simulation, we found, in fact, in each 1-minute interval, the disk is idle 57-99% of the duration of the interval, with a median of 82%. If the request rate in the trace doubles, the disk is idle less of the time, with a median of 60%. In the busiest periods, it is idle only 1%. Note that when the request rate doubles, the disk is almost saturated in the busy intervals (idle time drops to 1%), while the disk remains idle in the low-activity intervals. This idleness study used assumptions that represent Hummingbird: the disk has 64 KB blocks, all new files are written to the disk, and only data needs to be written to disk (no meta-information). The existence of idle periods is crucial for the design of Hummingbird since Hummingbird performs background bookkeeping operations in those idle periods (see Section 3.5).

Liddy Shriver 2001-05-01