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As density doubles with the rapidly dropping price each year for the past seven years (currently 32 Gbits/chip), NAND flash memory has virtually replaced HDDs (hard disk drives) in battery-operated consumer devices such as cellular phones, PMPs, and PDAs. This trend has also enabled the introduction of so-called flash memory SSDs (solid state disks) that have an interface identical to that of HDDs but use NAND flash memory inside as storage media. The ordinary filesystems designed for HDDs are no longer suitable for SSDs because SSDs have many different features from HDDs. We designed a hybrid filesystem, called HybridFS, that uses two kinds of storages - HDDs and SSDs. This is accomplished by distributing data into two partitions based on their type. In HybridFS, the data blocks of a large regular file are stored in a data partition in HDDs, while the metadata being stored in the partition of SSDs. Separating data into the different storages of disk partitions makes it possible to produce high I/O performance by taking appropriate I/O approach,according to the data characteristics.