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While the data size is growing and so are the storage capacities, the disk access speeds to write data to disk and read data from it is not keeping pace. Typical above-average current-generation 1 TB disks claim to access data at the rate of 300 Mbps, rotating at the speed of 7200 RPM. At these peak speeds, it takes about an hour (at best 55 minutes) to access 1 TB of data. With increased size, the time taken only increases. Besides, the claim of 300 Mbps at 7200 RPM speed is itself misleading. Traditional rotational media involves circular storage disks to optimize surface area. In a circle, 7200 RPM implies different amounts of data access depending on the circumference of the concentric circle being accessed. As the disk is filled, the circumference becomes smaller, leading to less area of the media sector being covered in each rotation. This means a peak speed of 300 Mbps degrades substantially by the time the disk is over 65 percent full. Solid-state drives (SSDs) are an alternative to rotational media. An SSD uses microchips, in contrast to electromechanical spinning disks. It retains data in volatile random-access memory. SSDs promise faster speeds and improved “input/output operations per second (IOPS)” performance as compared to rotational media. By late 2009 and early 2010, companies like Micron announced SSDs that could provide access speeds of over a Gbps (www.dailytech.com/UPDATED+Micron+Announces+Worlds+First+

Native+6Gbps+SATA+Solid+State+Drive/article17007.htm). However, SSDs are fraught with bugs and issues as things stand and come at a much higher cost than their rotational media counterparts. Given that the disk access speeds cap the rate at which you can read and write data, it only make sense to spread the data out across multiple storage units rather than store them in a single large store.

Source of Information : NoSQL