External mergesort is normally implemented so that each run is stored contiguously on disk and blocks of data are read exactly in the order they are needed during merging. We investigate two ideas for improving the performance of external mergesort: interleaved layout and a new reading strategy. Interleaved layout places blocks from di erent runs in consecutive disk addresses. This is done in the hope that interleaving will reduce seek overhead during merging. The new reading strategy precomputes the order in which data blocks are to be read according to where they are located on disk and when they are needed for merging. Extra bu er space makes it possible to read blocks in an order that reduces seek overhead, instead of reading them exactly in the order they are needed for merging. A detailed simulation model was used to compare the two layout strategies and three reading strategies. The e ects of using multiple work disks were also investigated. We found that, in most cases, interleaved layout does not improve performance, but that the new reading strategy consistently performs better than double bu ering and forecasting.
Zheng and Larson introduced a new reading strategy for external mergesort that consistently performs better than double buffering and forecasting REF .
Speeding up External Mergesort
IEEE Transactions on Knowledge and Data Engineering
Computer Science