PARTITIONS-General Info

From: Mike with contributions from Richard Sharpe
Revised Jan 2000

SEE LILO

PARTITION STRUCTURE FOR AN IDE HARD DISK and the MBR

There are three types of partition structure on an ide hard disk.

Physical
Extended
Logical

Rule #1 PRIMARY partitions: /dev/hdX1 to/dev/hdX4

There can only ever be four (4) primary partitions.

The first sector of each HD is called the MBR, or master boot record,  its structure is as follows:

 446 bytes are the master bootstrap.
  64 bytes are the four partition table entries (4 x 16)
   2 bytes are a signature ...
----
 512
----

Each exists as a permanent entry but indicates zero bytes used when not utilised.

These partitions correspond to hdX1,2,3 and 4, they cannot be called anything else.

Rule # 2 EXTENDED partition

One of these 'primary' partitions, and only one, can be instead an 'extended' partition. (which is not a partition at all). It points to a location on the disk which contains a linked list of 'logical' partitions.

*Any* of the four partitions hdX1 through 4 can be an extended 'pseudo' partition and is generally indicated as such in listings as

<hda2> hda5

This syntax says, the first logical partition hda5 is in the extended partition called hda2.

Rule #3 LOGICAL partitions

Logical partitions start at hdX5 and grow from there.

The reason why only one extended partition exists (if it exists at all) is because more than one is silly, it could only ever duplicate the linked chain present in the first one.

Theoretically, you can have as many logical partitions as you care to endure but the practical limit for DOS are 26-3 letters of the alphabet. For linux, the limit is 64-4 due to the make up of major and minor nodes in /dev directory that point to these ide partitions, and for SCSI, the limit is 16-4 for similar reasons.

NOTE: This idea of practical is a guide only. The linux OS for instance is developing so rapidly that it's numbers may have altered.
 

searchSearch Index