I'm reading the docs over at CentOS.org.
In section 25.1.2. Partitions: Turning One Drive Into Many, there is the following statement:
The partition table is divided into four sections or four primary partitions. A primary partition is a partition on a hard drive that can contain only one logical drive (or section). Each section can hold the information necessary to define a single partition, meaning that the partition table can define no more than four partitions.
I don't understand why there can only ever be four partitions. Is this just the way it was designed in the beginning? Can there really only ever be 4 primary partitions?
Best Answer
Yes, that's exactly it. The partition table at the front of an MBR disk (as opposed to a GPT style disk) has a very strict data-structure that dates from the 1980's when space was a precious, precious thing. The design decision way back then was to only allow four partitions, but allow one of them to be an 'extended' partition that was a pointer to another spot on the disk that could contain a lot more 'logical' partitions.
(This is the same reason why MBR formatted disks have trouble with 2TB+ disks. 512 byte size clusters, and 32bit fields containing cluster-counts for partition size = 2TB maximum disk size. A 4KB cluster size punts the problem down the road a ways.)
GPT is an updated method of handling partitioning that doesn't have these limitations.