become increasingly important, not only in mobile devices such as laptops but also in server and desktop markets. Increasing data center machine density has led to problems delivering sufficient power to devices (especially for spin-up), and getting rid of the waste heat subsequently produced, as well as environmental and electrical cost concerns (see green computing). Most hard disk drives today support some form of power management which uses a number of specific power modes that save energy by reducing performance. When implemented, an HDD will change between a full power mode to one or more power saving modes as a function of drive usage. Recovery from the deepest mode, typically called Sleep where the drive is stopped or spun down, may take as long as several seconds to be fully operational thereby increasing the resulting latency. The drive manufacturers are also now producing green drives that include some additional features that do reduce power, but can adversely affect the latency including lower spindle speeds and parking heads off the media to reduce friction. Other The or command overhead is the time it takes for the drive electronics to set up the necessary communication between the various components in the device so it can read or write the data. This is of the order of 3 μs, very much less than other overhead times, so it is usually ignored when benchmarking hardware. The is the time it takes the heads to settle on the target track and stop vibrating so they do not read or write off track. This time is usually very small, typically less than 100 μs, and modern HDD manufacturers account for it in their seek time specifications. Data transfer rate The data transfer rate of a drive (also called throughput) covers both the internal rate (moving data between the disk surface and the controller on the drive) and the external rate (moving data between the controller on the drive and the host system). The measurable data transfer rate will be the lower (slower) of the two rates. The sustained data transfer rate or sustained throughput of a drive will be the lower of the sustained internal and sustained external rates. The sustained rate is less than or equal to the maximum or burst rate because it does not have the benefit of any cache or buffer memory in the drive. The internal rate is further determined by