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while studying for my MCDST cert I learned about Primary partions and extended partitions. From the textbook:

****Primary A primary partition can be set as the bootable partition. A computer running a Windows operating system can have up to four primary drives (three if you also have an extended partition on the disk). Any primary partition may be configured as the active, or bootable, drive, but only one primary partition can be active at any time. When you are configuring a multiple boot computer, you will create a primary partition for each operating system and then install each operat­ ing system onto a different primary partition.

****Extended An extended partition provides a way to get around the four primary partition limits. You cannot format an extended partition with any file system. Rather, extended partitions serve as a shell in which you can create any number of logical partitions.

My question is: What are the differences? If an extended partition allows you to bypass the 4 primary partition limit, why have primary partitions at all?

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4 Answers

up vote 3 down vote accepted

In the beginning there was the original IBM PC BIOS partition scheme, which allowed up to four partitions. The FAT16 file system as used by MSDOS 2.x and 3.x would only support a formatted volume of 32MB. When a PC hard disk was 20MB this wasn't an issue but the advent of much larger ESDI(1) disks in the mid 1980s drove an urgent need for a scheme that could support more partitions.

The key issue here is backward compatibility. Older BIOS and DOS versions only supported the four partition scheme, so a partitioning scheme that would not break older versions but could support more partitions was needed. The solution was to allow partitions to be further sliced up by setting up another partition table within that partition.

Thus, the 'extended' partition was born. Extended partitions are just a format for slicing up a partition into sub-partitions. It exists solely as a kludge to preserve backward compatibility with the old partition format.

  1. ESDI stands for 'Enhanced Small Device Interface'. It was a refinement on the ST-506 interface that allowed more sectors on the disk, faster data transfer and also removed the artificial 1024 cylinder limit of the ST-506 interface. In 1982-3 a typical PC hard disk might have been about 20-30MB where you had one at all (they were quite expensive at the time). By 1986 you could purchase an ESDI hard disk with capacities up to about 330MB.
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thanks! - how do you guys know all this stuff? –  veggiemexicalidog Jul 9 '09 at 15:34
    
Age and decreptiude ;-} –  ConcernedOfTunbridgeWells Jul 9 '09 at 15:36
    
"By 1986 you could purchase an ESDI hard disk with capacities up to about 330MB" For a ridiculous fee at the time. –  Jim B Jul 9 '09 at 16:13
    
Even a 20-30MB drive of the era was more expensive than the PC it went in. –  ConcernedOfTunbridgeWells Jul 9 '09 at 21:15
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Simply put you need a primary partition in order to boot. Extended partitions cannot be the boot partition (beacuse it's just a container to get around the old limitations

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So that explains the reason you need one primary - but why the other 2? –  veggiemexicalidog Jul 9 '09 at 14:46
    
Originally you could only have the four primary partitions and the "extended" option was added later. You don't need the other two primaries) if you use the extended partitions (or the other three if you are not needing to boot from that drive). –  David Spillett Jul 9 '09 at 15:22
    
Other primary partitions can be create should OS'es require their own boot partition. The bootloader on the first primary partition (by default) can be configured to allow you to select one of the 4 possible primary partitions to continue the boot process from. Typically you might have set up 3 primarys to run 3 different OSes with the 4 partion an extended partion with one more partitions formated in the flavor of the day. –  Jim B Jul 9 '09 at 16:12
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All this "primary" and "extended" nonsense is based on old, old, old disk partition structures from way back in the day.

Basically, the master boot record (MBR -- a data structure stored on the first sector of the hard disk drive) has "slots" for up "pointers" that describe the on-disk location of up to 4 partitions.

Somebody got the idea that an entry in the master boot record, instead of "pointing" to a partition, could "point" to yet another list of up to 4 partitions. This is the "extended" partition table.

[MBR]
 |
 |--> Location on disk of partition #1
 |
 |--> Location on disk of partition #2
 |
 |--> Location on disk of partition #3
 |
 |--> Pointer to "extended" partition table
        |
        |--> Location on disk of partition #1
        |
        |--> Location on disk of partition #2
        |
        |--> Location on disk of partition #3
        |
        |--> Location on disk of partition #4

So, basically, all these arbitrary limitations you're learning to deal with are because we're nursing a data structure from the 1980's along. >smile<

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The extended partition is a primary partition, that can hold other partitions.

If you have an extended partition, you can only have three other primaries becasue teh extended itself adds one to the count.

The partitioning arrangement on PCs is mainly there due to history - there are other options these days (such as LVM under Linux and other unix-alike OSs or dynamic volumes under Windows).

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