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The hybrid drive, which combines the cost advantage of rotating magnetic disc storage with the high performance of NAND flash memory, has recently been attracting attention from computer users looking for fast, large capacity drives. The combination of a hard disk drive (HDD) and NAND flash memory has the potential to deliver a solid-state drive (SSD) like user experience. Hardware, firmware and architecture design are all essential to creating a product that will meet user expectations in terms of performance, capacity and cost. In the case of the latest technology, for example, that includes developing a new cache algorithm that places frequently used data into NAND flash memory, resulting in a hybrid drive with SSD-like performance.

1. INTRODUCTION
Large numbers of video and other types of data are being handled more frequently than ever before. People want to store more data on devices installed in PCs (e.g., HDDs), yet still achieve increased data access performance. For example in operating system (OS) data readout performance for quick booting PCs, as well as improved data read/write performance while PCs are in operation. HDDs are still the major player in the storage device arena and are used in numerous fields, though there are limitations on performance improvement potential. For this reason, SSDs, which feature high-speed data access, are attracting attention and the SSD market is expanding. However, HDDs still have the advantage in cost per gigabyte.

Given these circumstances, Hybrid drives, which combine HDDs with NAND memory, have come into their own as storage devices capable of realising both high capacities and high performance. Now, by adopting an intelligent cache algorithm that learns what data should be stored in the NAND memory cache, the latest generation of these drives is set to deliver HDD capacity and SSD-level performance to both enterprises and consumers.

2. WHY CONSIDER HYBRID DRIVES?
As the amount of data created increases in tandem with the popularity of sharing data via the internet, the capacity of storage devices such as the HDDs used in PCs is also increasing. Still, more and more SSDs are being employed in notebook PCs because of the advantages they offer in terms of increased data access speeds, decreased PC start-up times, compact size, and low power consumption.

The Hybrid drive achieves both a high capacity and high performance by combining traditional NAND memory technologies, NAND memory handling technologies (instrumental in SSD technologies), and HDD product development technologies.

Figure 1 shows a forecast of what types of built-in storage devices will be used in notebook PCs. Full-fledged adoption of hybrid devices, which simultaneously realise both high capacities and high-speed data access performance while keeping bit costs low, is estimated by IDC to account for about 25% of the market in 2015.

3. ARCHITECTURE
The basic structure of a Hybrid drive is shown in Figure 2. For the cache memory, the HDD uses low-capacity, high- speed DRAM (volatile memory) and a large-capacity magnetic disk (non-volatile memory). NAND memory (non- volatile memory), which has medium capacity, read/write speeds, and bit costs compared to DRAM and magnetic disks, is added as a secondary cache in this HDD.

A newly developed cache algorithm dynamically studies data access patterns and stores frequently accessed data in non-volatile NAND memory. This allows access performance to be improved even if no cache hits occur in DRAM during use, for example when booting the PC. Thus, the algorithm can help the hybrid drive achieve performance levels close to that of SSDs.

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