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Hard Disk And File Systems

Appendix A. Hard Disk And File Systems

These appendices contain additional information about hard disk construction and data storage, partitions, file systems and interaction of operating systems with hard disks.


Hard Disk Organization

All hard drives have basically the same structure. Inside the case, there are several disks with a magnetic coating set on a single axis (spindle). A special motor provides the necessary rotation speed to the spindle, e.g. 5,400 rpm, 7,200 rpm, or 10,000 rpm.

Information on disks resides on concentric tracks. Each track has a number. The outermost track is number 0, and the numbers grow inwards.

Each of the tracks is divided into sectors that contain minimal information blocks that can be written to the disk or read from it. Sectors also have numbers. On every disk, there is a marker that indicates the beginning of sector enumeration. The sector that is the closest to this marker is number 1.

At the beginning of a sector, there is a header (prefix portion) that marks the beginning of the sector and its number. At the end of a sector in the suffix portion, a checksum is used to check data integrity. The data area between the prefix and suffix portions is 512 bytes in size.

Both sides of each disk on the spindle are used to store data. All tracks that have the same number on all the surfaces of all disks comprise a cylinder. For each work surface of a disk in the drive, there is a head that enables reading and writing data from/to the disk. Heads are assembled into a block and are numbered, starting with 0.

To perform an elementary read or write operation, the head block should be positioned at the necessary cylinder. When the appropriate sector (with the appropriate number in the service area) of the rotating disks approaches the head, data is exchanged between the head and the electronic circuit board of the disk drive.

Sector structure of a hard disk is created via low-level formatting during which each of the tracks of the disk is marked up. This process generally takes place when the drive is manufactured.

Modern disk drives usually contain relatively few magnetic disks (1–2) to make the head block lighter and speed up access to sectors (a drive like this has 2–4 heads respectively).

There can be up to several tens of thousands of cylinders per disk. The greater the amount of data that can be stored on one side of a disk,, the more cylinders can be created on it and the larger the capacity of the disk.

This design has a lot of technical implementation peculiarities, but those issues are not germane to this explanation.

Copyright © Acronis, Inc., 2000–2010


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