Partition-Mass-Storage-Dummies-Linux-HOWTO
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# 3. Definitions

## 3.1. Disks

Disks are usually made of rotating plates, read by magnetic heads. Tracks are circular parts of the plates. As we may have several plates, a stack of plate is seen as a cylinder. All the heads are moved at the same time, reading each it's track. All the tracks read at the same time are a cylinder as well.

Each track is divided as sectors that can be 512 or 4k bytes long.

So Mass Storage disks are nearly always described as "CHS", that is Cylinders number, Heads number, Sectors by track number, and the product of all these numbers gives the visible disk size.

However, this have no meaning for SSD and even with true rotating hard drive, the CHS have absolutely no more any meaning! It keeps using only by inertia... so don't worry too much about these numbers, and if possible ignore them.

## 3.2. Partitions

Partition is from "part". A partition is a part of a drive with some special attributes.

Computers makers seems pretty dumb when seeing how they keep underestimating the possible next disk size, so each year they have to issue a new standard. Disks firmware are buggy, so the software tools have to fix them - and do quite well the job.

That is to say that understanding fully why the Partitionning tool choose to create the partitions the way they do is very complex. It's explained fully (or mostly) on the Partitions-and-mass-storage-HOWTO, you probably won't read and don't have to now.

Windows makes little use of partitions. One can install as many Windows version he wants on the same partition (and often do). It may even proove difficult to install Windows on just an other drive on the same computer.

Nor Unix nor Linux have any such problem. Just on the contrary, Linux love to use at least three partitions. This mean you can have as many Linux on your drive as you want, each on it's special part(ition) of the drive, erase the partition without losing your data (backup first is still better) for example. The two main Linux partitions are used for the system ("/" or "root") and the users data ("home"). The third ("swap") is used to add some more memory and is written directly sector by sector by the system. You usually don't even see it.

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