I would like to know

What is the exact meaning of primary partitions? Why it is named so? and why it is restricted to 4?

What is meant by extended partitions? Why it is named so? and what is the possible number of extended partitions in the hard disk?

What is mean by logical partitions? Why it is named so? How it is calculated?

What are the advantages of these software partitioning?

Is it it possible to install OS(Linux/windows) in all partitions ? If no, why?

  • Please don't moderator flag questions because it doesn't get answers. The moderators have no influence on questions getting answered. – BinaryMisfit Oct 21 '10 at 7:20

The "MS-DOS" partitioning scheme is limited to 4 primary partitions. This is all that will fit in the first sector (512 bytes) of the disk after the MBR (460 bytes).

In order to have more than 4 partitions, one of them becomes an extended partition, which points to an extended structure further down the disk. This structure lists the logical partitions on the disk.

The "legacy" MBR uses one of the flags on the primary partitions to determine which to boot up; this flag does not exist on logical partitions, so they cannot be booted the same way. Other MBR replacements (e.g. GRUB) do not use this flag and so can boot logical partitions.

Theoretically, any OS can be installed to either primary or logical partitions, but older versions of Windows will not boot properly if not installed to a primary partition.

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  • Can you little bit deeper ? that is about the first boot sector-- 512 bytes? among that 460 bytes MBR then 52 bytes for 4 partitions? how the logic behind that? The first boot sector(512 bytes) also in the first primary partition right? How we configure the 52 bytes for 4 partitions? Why MBR takes 460 bytes , what it contains? – Renjith G Oct 15 '10 at 22:50
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    The legacy MBR contains the logic needed to detect which of the 4 primary partitions has the boot flag set as well as the instructions needed to tell the computer to load from that partition and start it. 13 bytes was all that was needed for each partition, so that's what they gave them. They figured 4 partitions would be plenty, so they let the MBR have the rest of the space. – Ignacio Vazquez-Abrams Oct 15 '10 at 22:54
  • not at all clear! :-( .... there should be a superb bit logic behind that! – Renjith G Oct 15 '10 at 22:58
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    It was logical at the time they invented the 512 boot sector, they never figured anyone would need more partitions at the time, kind of like tcp/ipv4, the never figured we would ever need more addresses at the time, but we do now! – Moab Oct 16 '10 at 18:01
  • Ok. But i want the bit division logic of that 512 bytes boot sector ! – Renjith G Oct 18 '10 at 4:16

Partitions in first place are required for these two reasons-

  • To arrange and maintain the data in an easier way

  • To install one or more OS in your system.

Each partition can have only one OS installed in it. Now coming to the primary partitions part and why they are only 4, we need to look at Master boot record or MBR. MBR has a size of 512 bytes. First 446 bytes consist of Initial program loader(IPL) which are essential for the boot process. Next 64 bytes consist of the partition table. Remaining 2 bytes are the magic bytes which point to the boot files on the hard disk. Each partition that is made is recorded in the partition table. Each partition entry in the partition table needs 16 bytes. Hence 64 bytes table allows us to make only 4 primary partitions.

If we need more than 4 partitions, we move towards extended partitions. Since these partitions are extended form of primary partitions, hence the name.

Only one extended partition can be made. Three other partitions would be primary partitions, because anyhow the partition table would be able to handle only 4 partitions, so there would be 3 primary and one extended. Inside extended partitions we make logical partitions. As such, there is no limit to the number of logical partitions. In some books it is mentioned that logical partitions have a limit of 128, but I've personally made more than that on RHEL 7. There can be many number of logical partitions until the size allocated to extended partition gets exhausted.

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