Basic disk structure


You've probably seen pictures of disk layouts before, describing the disk as a pie. Far be it for this book to be different; think of a disk surface divided up into slices ” like a pie. As the disk spins , a read head sitting in one position will describe a circle over the surface, entering into each slice. The data in a slice, known as a sector, is the basic element of data storage on a disk surface. This is what a read or a write deals with. Fine so far, but eventually you're going to run out of room on this one circle, and you'll need to move to a new one. Since most disk drives involve more than one disk surface, in fact containing many two-sided platters, it turns out that they have multiple read/write heads stacked on top of each other, one for each surface. This gives you a bunch of stacked circles, known as cylinders , to play with. Sooner or later, though, this, too, will fill up, and it will be time to move on.

The next step is to move to an adjacent circle, or cylinder. Most disks have hundreds of cylinders available. However, there is a disadvantage in going to another cylinder to find data: The disk head must move. This operation, known as a seek , is the most expensive operation in disk I/O. Much of the work in optimizing disk layouts for faster access has been in minimizing the number of seeks performed, or at least keeping them as short as possible.



PANIC. UNIX System Crash Dump Analysis Handbook
PANIC! UNIX System Crash Dump Analysis Handbook (Bk/CD-ROM)
ISBN: 0131493868
EAN: 2147483647
Year: 1994
Pages: 289
Authors: Chris Drake

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