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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk Internal Performance Factors | Data Recording and Encoding Factors ]

Track and Sector Layout

There are several effects on performance that come about as a result of how the data on the surface of the platter is organized. In order to make sense of a platter surface that can store 10 GB of data or more, there has to be a way of organizing it into smaller, more manageable pieces. To accomplish this end, each surface is split into tracks, and then each track is further split into sectors, each sector holding 512 bytes of user data (normally). The track and sector layout of the hard disk, and some of the issues in how this organizing is done, are discussed here.

The most important impact of the track and sector layout is on sustained transfer rate. The various techniques used in improving the way data is organized on the hard disk all are primarily oriented around increasing the amount of data that can be stored in a given amount of space, and this mainly improves sustained transfer rate. Some advances also improve positioning speed. Here's a quick list of the performance enhancements that fall into this general category:

  • Optimal Interleaving: All modern drives use optimal (1:1) interleaving, which cannot be changed. Thus, this factor is not really relevant for distinguishing modern drives. However, you should understand that proper interleaving does ensure that the maximum possible transfer rate is realized on a drive. Older drives that were sometimes set to the wrong interleave factor would have greatly reduced transfer rates compared to their potential maximums.
  • Zoned Bit Recording: The use of zoned bit recording has allowed the larger outer tracks of a hard disk to be used to their full potential. It is also the reason that the media transfer rate of a disk depends on what part of the disk is being accessed; since outer tracks have more sectors, they have a higher transfer rate.
  • Cylinder and Head Skew: Optimal cylinder and head skew factors built into the drive controller are necessary for high sustained transfer rates; they optimize cylinder switch time and head switch time respectively.
  • Sector Format: Improved sector formats and higher sector format efficiency allow a larger percentage of a track to contain data, increasing capacity and transfer rates. Also, the "no ID" sector format improves random positioning by saving time during random seeks, and avoiding "detours" around remapped sectors.

Next: Controller and Cache Factors

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