Looking at how hard disk works.

enter image description here

Found that it has a number of platters and a read/write head. But in texts it shows read/write head between each platter. Is it like that in real situation?

Also when I give the following command in Linux

user@user:~$ sudo fdisk /dev/sdb
[sudo] password for user: 

The number of cylinders for this disk is set to 30401.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): p

Disk /dev/sdb: 250.1 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

It shows 255 heads, 63 sectors/track, 30401 cylinders. I couldn't understand the significance of the cylinders. Could someone explain on that.

Also it lists 63 sectors/track.

What does it mean. i understand that tracks are divided into sectors. But what does the above entry mean?

  • ive taken apart a few average HDD's but i think i would have noticed 255 heads... its also a perfect 8bit number and that raises all kinds of suspicion in my head.... – Silverfire Sep 27 '11 at 4:46

Yes, there is a head facing each data surface. Otherwise, there would be no way to read from and write to that surface. All the heads are physically connected to the same actuator, so they can't move independently. (A three-platter drive will typically have six heads.)

When you see the reported numbers of heads, sectors per track, and cylinders, you are actually seeing the drive's logical arrangement. It doesn't necessarily have any simple relationship to the drive's physical arrangement. (Do you really think the drive has 255 heads?)

Modern operating systems and even BIOSes use strictly 48-bit linear logical block addressing. So the information you are seeing there no longer means anything. (It was relevant back in the day when the controller and the drive were separate components. Now drives just report the maximums for all those fields.)

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