Monday, June 11, 2012

A Guide to Laptop Hard Drives

Laptop hard drives are smaller variants of their larger desktop counterparts and generally emphasize power efficiency more than raw capacity of groundbreaking performance. There are fair a number of major industry stalwarts competing in the cutthroat laptop hard drive business: Fujitsu, Hitachi, Samsung, Seagate, and Western Digital.

Past generations of laptop hard drives relied on parallel ATA interfaces which not only limited transfer bandwidth, but were not as electrically efficient as the power sipping SATA and SATA 3.0 Gpbs standards that saturate the market today. The interface is a key performance factor of all laptop hard drives as it is a measure of the maximum amount of data that can be transferred to and from the drive. While the interface standards for laptop hard drives has kept well ahead of their sustained performance potential, it is not uncommon for the interface to be a limiting factor for transfer performance in quick bursts.

Another performance characteristic, often considered to be the primary factor influencing overall read/write speed as well as access time is the rotation velocity. Laptop hard drives have power and thermal constraints that limit their performance to spindles with speeds of 7200 rotations per minute (RPM) or less. The faster a given disk can rotate its magnetic platter(s), the faster it can write information as well as find and read stored data. Even at 7200 rpm, the performance of laptop hard drives does not measure up to that of similar capacity desktop units.

One of the major reasons for this is the number of platters is limited by the physical height of the drive. Magnetic platters that store data in any hard drive need a certain amount of space between them in order for a read/write head to move across them and create, change, or access information. Laptop hard drives are physically smaller, and thus the amount of platters is limited, but so is the performance per platter. Take any two spinning circles of different sizes and one will note that when rotating the same number of times in the same period, the outer edges of the larger circle will travel a greater total distance. In the world of laptop hard drives this means that the distance a read/write covers at 7200 rpm is quantifiably less than the distance it would cover if the platter were physically larger.

The platters themselves also play a factor. The more densely packed the information, referred to as the aural density, the faster it can generally be read from and the quicker it can be written. This is not always true, as sometimes the technology for the read/write heads lags behinds and an increase in aural density results in slower seek times.

The final performance related factor for laptop hard drives is their cache. The cache is a small pool of volatile memory similar to system RAM that acts as a buffer for writes and often stores recently and/or frequently accessed information. The buffer is the primary reason why interface standards can limit bursting performance despite the lack of sustainable performance approaching theoretical limits of the interface specification.