Understanding Hard Drive Performance

Hard Drive Internals

Bits are organized in so-called sectors, which are combined into allocation units (clusters). A cluster is the smallest chunk of data to store data. Depending on the file system (Windows uses NTFS or FAT32) the cluster size may vary The larger a cluster, the better is the overall sequential throughput, but you will end up wasting storage capacity if the average file size is much smaller than the cluster size.

Form Factors And Height

The most obvious difference between hard drives is their form factor, which is based on the platter diameter. Hard drives for desktop computers use 3.5" platters, while mobile hard drives use a diameter of 2.5". Enterprise hard drives may look like 3.5" models, but they actually use smaller platter diameters to enable higher rotation speeds. Hard drives for ultra-portable devices have platter diameters of only 1.8", and there are various micro hard drives with only 1" and 0.8" platters as well.

Hard drives in the 3.5" form factor typically have a height of 1", which is sufficient to accommodate up to five platters inside the drive. Notebook hard drives use single or twin platter designs and adhere to 9.5 mm or 12.5 mm height limits, though the latter isn't suitable for most notebook designs. If you look at 1" and 0.8" hard drives you'll notice a tendency towards proprietary solutions and heights, because there products are often optimized for certain customer requirements.

More platters certainly yields a higher storage capacity, because the total capacity is calculated by multiplying the per-platter capacity with the number of platters. For example, a data density of 160 GB per platter allows manufacturers to reach 640 GB capacity per drive with four platters. However, more platters also mean more read/write heads, which increases the risk of hardware failure due to the larger number of moving parts. Friction and energy requirements increase as well. In terms of price, one high capacity drive is still less expensive than multiple smaller ones. The only exception is high performance RAID arrays in servers, which intentionally run more hard drives to increase performance.

Patrick Schmid
Editor-in-Chief (2005-2006)

Patrick Schmid was the editor-in-chief for Tom's Hardware from 2005 to 2006. He wrote numerous articles on a wide range of hardware topics, including storage, CPUs, and system builds.

  • t30ne
    why only comparing seagate? what about WD?
    Reply
  • darkfall13
    They were comparing the minute differences or in this case indifferences within a range of nearly identical hard drives. Branding really didn't matter except to just have them all the same...
    Reply