I have basic understanding of disk. (Please correct me if i am wrong) I understand on 1 platter, there are 2 heads, 1 on top, and 1 on bottom. Both side of the platter is able to provide storage ? On the platter, there are many tracks being broken up into the smallest unit of storage = sector. Corresponding tracks across all the platters = cylinder.

In OS, a block/cluster can be 1 or more sector


This is my output of fdisk -l

Disk /dev/sda: 107.4 GB, 107374182400 bytes

255 heads, 63 sectors/track, 13054 cylinders

Units = cylinders of 16065 * 512 = 8225280 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x0000d866

From the output, can i say that there are

q1) is there a total of 13054 tracks per platter then ? since there are 13054 cylinders ?

q2) there are 2 sides to a platter, does a track or sector applies to 1 side of a platter or both sides ? (e.g. if there is 1 track on platter, does it literally means 2 track, 1 on top and 1 below ?)

q3) what does the UNITS calculation represent ? Why isn't the total storage in a disk = no. of platter (not sure if top and bottom counts) * no of tracks per platter * no of sector per track * size of sector ?

q4) What does 255 heads means ?


1 Answer 1


Let's start with this one:

q4) What does 255 heads means ?

"255 heads" means that you're looking at made-up numbers.

(Also, your fdisk version is rather out-of-date).

Modern disk drives haven't used cylinder-head-sector addressing for almost two decades, preferring logical block addresses over CHS.

One reason for that is that modern disks are simply too large to be described with the C/H/S scheme, which is limited to 1023/255/63 (in some places) or 65535/16/255 (in other places), which is 128 GB at most.

Another is that the disk's own controller can make better decisions than the OS about where exactly to put data. Finally, SSDs don't even have "cylinders" nor "heads" nor even "platters".

For these reasons, the reported CHS numbers are really meaningless. (255 heads wouldn't even fit inside a regular 3.5" disk.) The OS only knows the sectors by a single linear address – in your case, from 0 to 209715200 – and the disk controller translates it internally.

That said:

On old disks, which actually had a CHS geometry, yes, both sides would have their own tracks, and they would report one 'head' per side.

Naturally, a platter has 2 sides and thus 2 surfaces on which data can be manipulated; usually there are 2 heads per platter, one per side. [Wikipedia]

Continue your research at:

  • thanks for the detailed explanation. Since CHS is no longer in use, why does fdisk output still somehow follow the format ? I have also not understand why does it mentioned that there is 13054 cylinders and yet the calculation is 16065 cylinders x 512 bytes instead. what does 16065 represents ? no of sectors ? why is it multiplying with 512 bytes directly ? how does all these figures make up 107374182400 bytes ?
    – Noob
    Jun 29, 2015 at 17:52
  • @Noob: It's multiplying with 512 bytes because one sector is 512 bytes large on your disk. But the figures don't necessarily make up 107374182400 bytes. CHS can't describe disks that large. fdisk only shows the CHS information because you have an old version of fdisk. Jun 30, 2015 at 6:36
  • thanks for the reply. are you referring that 16065 is the number of sectors of all the tracks in the platters ? But it is already stated that there are 13054 cylinders which implied that there are 13054 tracks per side of a platter. It is also stated that there are 63 sectors per track. So how does the figure 16065 came about ?
    – Noob
    Jun 30, 2015 at 14:11
  • 16065 is 255 × 63. But there aren't 63 sectors per track. There are thousands of sectors per track, and different tracks belong to different zones which have different numbers of sectors, and CHS cannot possibly describe that. The OS still has to report something when asked about the CHS data – it cannot give all-zeros – so it just gives you the largest numbers it can, but that doesn't change the fact that the numbers do not make sense and trying to make sense of them is a complete waste of time. Jul 1, 2015 at 6:37

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