Hard Disk Sector Structures

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A sector is the basic unit of data storage on a hard disk. The term "sector" emanates from a mathematical term referring to that pie shaped angular section of a circle, bounded on two sides by radii and the third by the perimeter of the circle - See Figure 1. An explanation in its simplest form, a hard disk is comprised of a group of predefined sectors that form a circle. That circle of predefined sectors is defined as a single track. A group of concentric circles (tracks) define a single surface of a disks platter. Early hard disks had just a single one-sided platter, while today's hard disks are comprised of several platters with tracks on both sides, all of which comprise the entire hard disk capacity. Early hard disks had the same number of sectors per track location, and in fact, the number of sectors in each track were fairly standard between models. Today's advances in drive technology have allowed the number of sectors per track, or SPT, to vary significantly, but more about that later.

 Figure 1

When a hard disk is prepared with its default values, each sector will be able to store 512 bytes of data. Without elaborating, there are a few operating system disk setup utilities that permit this 512 byte number per sector to be modified, however 512 is the standard, and found on virtually all hard drives by default. Each sector, however, actually holds much more than 512 bytes of information. Additional bytes are needed for control structures, information necessary to manage the drive, locate data and perform other functions. Exact sector structure depends on the drive manufacturer and model, however the contents of a sector usually include the following elements:

• ID Information: Within each sector a small space is left to identify the sector's number and location, which is used to locate the sector on the disk and provide for status information about the sector itself. For example, a single bit is used to indicate if the sector has been marked defective and remapped.

• Synchronization Fields: These are used internally by the drive controller to guide the read process.

• Data: The actual data in the sector.

• ECC: Error correcting code used to ensure data integrity.

• Gaps: Often referred to as spacers used to separate sector areas and provide time for the controller to process what it has been read before processing additional data.

• Servo Information: In addition to the sectors, each of which contain the items above, space on each track is allocated for servo information on drives that utilize embedded servo drives. Most, if not all, modern drives not employ servo technology.

Each of the elements identified above are collectively known as sector overhead. The amount of space taken up by this collective overhead is important, as the more bits that are relegated to disk or data management, the fewer that will be available for actual data storage. Hard disk manufacturers strive to reduce the amount of overhead information actually stored on the disk. When you see the term format efficiency, it refers to the percentage of bits on each disk that are used for data, as opposed to overhead. Obviously, the higher the format efficiency the better, however this information is closely held by disk manufacturers and extremely difficult to track down.

One of the most important innovations in sector format was IBM's 1990 development of the No-ID Format, whereupon the ID fields are removed from the sector format. Instead of labeling each sector within the sector header, a format map is created and stored in memory and referenced when a sector must be located. This map contains information about which sectors have been marked bad and data relocated, sector location relative to the location of servo information etcetera. This improves format efficiency by allowing up to 10% more data to be stored on the surface of each platter, while improving performance by eliminating the need to extract this information from each sector. Critical positioning information is present in system memory, therefore it can be accessed much more quickly.

Zoned Bit Recording

We would be remiss in our discussion of drive sectors, tracks and performance without mentioning mass improvements such as Zoned Bit Recording. One of the methods used to increase capacity and data access speeds on hard disks is by improving the utilization of the larger, outer tracks of the disk. Early hard disks were extremely primitive, and their controllers weren't capable of handling complicated arrangements such as being able to change tracks. As the result of this arrangement, every track had the same number of sectors, with the standard set at 17 sectors per track.

As you can see from our sketch above, Figure 1, tracks are concentric circles, with the ones on the outside of the platter much larger in circumference than the ones closer to the center. Since there is a constraint on how tightly the inner circles can be packed with bits, developers packed them tightly as possible given the state of technology at the time. By reducing bit density, developers were able to assign the same number of sectors to the outer circles. Essentially this meant that the inner sectors were being packed so tightly there was no room for error, and the outer sectors underutilized, as in theory they could hold many more sectors given the same linear bit density limitations as were imposed on the inner sectors.

Drive developers, in an effort to create larger drive sizes, as well as improve utilization and performance, developed a technology referred to as zoned bit recording (ZBR). Zoned bit recording is often referred to as  multiple zone recording or just zone recording. With this technology, tracks are grouped into zones based on their distance from the center of the disk, and each zone is assigned a number of sectors per track. As you move from the innermost part of the disk to the outer edge, you move through different zones, each containing more sectors per track than the one before. This makes more efficient use of the larger tracks on the outside of the disk. In essence, with ZBR, the size (or length) of a sector remains reasonably constant over the entire surface of the disk. Stark contrast to very early hard disks that did not employ ZBR, as their tracks were limited to only 9 sectors regardless of track size.

An interesting added benefit from zoned bit recording is that the raw data transfer rate of the disk, also referred to as the media transfer rate (a bit of a misnomer), when reading the outside cylinders is considerably higher than when reading the inside ones. Although the angular velocity of the platters is constant regardless of which track is being read, the outer cylinders contain more data. Bear in mind though that angular velocity does not necessarily compensate for the fact that the outer tracks (periphery of the platter) is moving much faster than the tracks at the core of the platter.

Take note that constant angular velocity is not the case for all drive technologies, such as older CD-ROM drives.

Since data is written to the outer tracks of a drive first, hence the drive is filled with data from the outside in. The fastest data transfer occurs when the drive is first used and data retained in the outer tracks. Many people that perform benchmarks on their systems and their hard drives when new, then make some tweaks and changes to their system only to return to their benchmarks weeks or months later only to be unpleasantly surprised that the disk and its benchmarks are getting slower. Actually, the disk has probably has not changed at all, but the second benchmark may have been run on tracks closer to the center of the disk. While most people that take benchmarking seriously defragment their drives before running the tests, fragmentation of the file system can have impact performance benchmarks.

Let's take a look at how just four years has changed drive performance. Below you will find two data tables, one from Quantum (now Maxtor) released in1996 and another from IBM released in 2000.

The table below (Figure 2) shows the zones used by a 3.8 GB Quantum FireballTM hard disk having a total of 6,810 usable data tracks on each platter. Also included is the raw data transfer rates for each zone. Notice how the transfer rate decreases as you move from the outer edge of the disk (zone 0) to the center of the disk (zone 14). The data transfer rate at the edge of the disk is almost double that of the center.

Figure 2
 Zone Tracks in Zone Sectors Per Track Data Transfer Rate (Mbits/s) 0 454 232 92.9 1 454 229 91.7 2 454 225 90.4 3 454 225 89.2 4 454 214 85.8 5 454 205 82.1 6 454 195 77.9 7 454 185 74.4 8 454 180 71.4 9 454 170 68.2 10 454 162 65.2 11 454 153 61.7 12 454 142 57.4 13 454 135 53.7 14 454 122 49.5

*Courtesy Maxtor/Quantum FireballTM Product Manual, © 1996

In the case of this specific drive, having the same number of tracks per zone was not a requirement, it just happens to be the manner in which Quantum designed this disk family. You'll see a difference when making a comparison to the newer IBM drive below. Notice the larger number of sectors per track as compared to the 17 sectors per track of earlier disks. Today's modern drives offer considerably more storage per track as well as even higher numbers of sectors per track in all zones, and much higher data transfer rates.

Here's the same chart for the 20 GB/platter, 5400 RPM IBM 40GV drive:

Figure 3
 Zone Tracks in Zone Sectors Per Track Data Transfer Rate (Mbits/s) 0 624 792 372.0 1 1,424 780 366.4 2 1,680 760 357.0 3 1,616 740 347.6 4 2,752 720 338.2 5 2,880 680 319.4 6 1,904 660 310.0 7 2,384 630 295.9 8 3,328 600 281.8 9 4,432 540 253.6 10 4,528 480 225.5 11 2,192 440 206.7 12 1,600 420 197.3 13 1,168 400 187.9 14 18,15 370 173.8

Courtesy IBM - Deskstar 40GV and 75GXP Product Manual, © 2000

As you can see, the number of tracks per zone, sectors per track and data transfer rates are all several times higher than the numbers for the older drive shown in Figure 1, and shows how dramatically rive capacity and performance have increased in just four years. While the number of tracks per zone differs for each zone, unlike the Quantum drive, the number of zones remains the same. Don't read anything into the fact that both drives have exactly 15 zones, as this is merely a coincidence. Some drives have more, while others have less, but most new drives tend to have the same number of zones as older ones. Although you can increase the number of zones, doing so makes the controller more complicated, and there really isn't any benefit in doing so. In a perfect set of circumstances, you could maximize the storage potential if every track were to have its own zone with a perfect number of sectors, however any additional storage capacity gained would be relatively small compared to just using larger zones as shown above in Figure 3.

Standard BIOS settings for IDE/ATA hard disks permit you to specify only a single number of sectors per track. Since all modern hard disks use zone bit recording, and do not have a single number of sectors per track, they use *logical geometry for the BIOS setup. IDE hard disks up to 8.4 GB usually report to the BIOS that there are 63 sectors per track and then translate to the true geometry internally.

No modern drive uses 63 sectors on any track, much less all of them.

When drive designers a operating system developers discovered the 8.4 GB BIOS limit, they realized that hard drives greater than 8.4 GB weren't capable of having their parameters expressed using any of the IDE BIOS geometry parameters. Therefore these drives would always report having 63 sectors per track as pseudo (false) geometry parameters, and then accessed using logical block addressing.

Unlike legacy computer systems, today's motherboard, and especially hard disk controllers must have intricate, detailed information about the data recording zones of a hard drive, which includes the number of sectors in each track, as well as the number of tracks in each zone. For this very reason, all drive manufacturers today low-level format their drives before they leave the factory.

Unlike older drives, 1998 and earlier (or 8.4 GB or smaller), it is not recommended that newer drives be low-level formatted unless the procedure is absolutely necessary, and specific low-level formatting tools for the specific drive in use are used.

*Logical Geometry - A generic term used to describe the manner in which a manufacturer has designed its disk to structure its data into platters, tracks and sectors, or its geometry. In early disk designs, this was relatively simple, as disks had a certain number of heads, tracks per surface, and sectors per track. These numbers were entered into the BIOS during set up so the system knew how to access the drive. Today, with newer drives, the situation is considerably more complicated.

You can read about Logical Block Addressing, or LBA, by clicking here.

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