Block-Based Addressing - Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

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this address — the head and sector — remain undefined.
5.2.1.2. Head
Although in the strictest sense we are selecting a particular disk platter, because each surface has a
read/write head dedicated to it, it is easier to think in terms of interacting with a specific head. In
fact, the device's underlying electronics actually select one head and — deselecting the rest — only
interact with the selected head for the duration of the I/O operation. All other tracks that make up the
current cylinder have now been eliminated.
Cylinder
1014
Table 5-2. Storage Addressing
In Table 5-2, the first two parts of a geometry-based address have been filled in. One final component
to this address — the sector — remains undefined.
5.2.1.3. Sector
By specifying a particular sector, we have completed the addressing, and have uniquely identified the
desired block of data.
Cylinder
1014
Table 5-3. Storage Addressing
In Table 5-3, the complete geometry-based address has been filled in. This address identifies the loca-
tion of one specific block out of all the other blocks on this device.
5.2.1.4. Problems with Geometry-Based Addressing
While geometry-based addressing is straightforward, there is an area of ambiguity that can cause
problems. The ambiguity is in numbering the cylinders, heads, and sectors.
It is true that each geometry-based address uniquely identifies one specific data block, but that only
applies if the numbering scheme for the cylinders, heads, and sectors is not changed. If the numbering
scheme changes (such as when the hardware/software interacting with the storage device changes),
then the mapping between geometry-based addresses and their corresponding data blocks can change,
making it impossible to access the desired data.
Because of this potential for ambiguity, a different approach to addressing was developed. The next
section describes it in more detail.

5.2.2. Block-Based Addressing

Block-based addressing is much more straightforward than geometry-based addressing. With block-
based addressing, every data block is given a unique number. This number is passed from the computer
Head
2
Head
2
Chapter 5. Managing Storage
Sector
X
Sector
12