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Although the technology of flash memory is rapidly improving, SSD usage is still limited due to the high cost per storage capacity. An alternative is to construct the hybrid structure where a small SSD partition is combined with the large HDD partition, to expand the file system space to HDD storage capacity while exploiting the performance advantage of SSD. In such a hybrid structure, increasing the space utilization of SSD partition is the critical aspect in generating high I/O performance. In this paper, we present HFM (Hybrid File Mapping) that has been implemented for the hybrid file system integrated with SSD. HFM enables to divide SSD partition into several, logical data sections with each composed of the different extent size. To minimize fragmentation overhead, HFM defines three different ways of partitioning functions based on the extent size of each data section. Furthermore, file allocations on the extent are performed on the partitioned unit (segment) of extents to reuse the remaining free space as much as possible. The experimental result of HFM using three public benchmarks shows that HFM can be effective in increasing the usage of SSD partition and enables to contribute to provide better I/O throughput.

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