Disk partition in dos, windows, linux, unix - 1

Disk partitioning

Disk partitioning is the creation of separate divisions of a hard disk. Once a disk is divided into several partitions, directories and files of different categories may be stored in different partitions. More partitions provide more control but too many become cumbersome. The way space management, access permissions and directory searching are implemented depends upon the type of file system installed on a partition. Careful consideration of the size of the partition is necessary as the ability to change the size depends on the file system installed on the partition.

Purposes for partitioning

• Separation of the operating system files from user files
• Having an area for operating system virtual memory swapping/paging.
• Keeping frequently used programs and data near each other.
• Having cache and log files separate from other files. These can change size dynamically and rapidly, potentially making a file system full.
• Use of multi-booting setups, which allow users to have more than one operating system on a single computer. For example, one could install Linux, Mac OS X, Microsoft Windows or others on different partitions of the same hard disk and have a choice of booting into any operating system (supported by the hardware) at power-up.
• Protecting or isolating files, to make it easier to recover a corrupted file system or operating system installation. If one partition is corrupted, none of the other file systems are affected, and the drive's data may still be salvageable. Having a separate partition for read-only data also reduces the chances of the file system on this partition becoming corrupted.
• Raising overall computer performance on systems where smaller file systems are more efficient. For instance, large hard drives with only one NTFS file system typically have a very large sequentially-accessed Master File Table (MFT) and it generally takes more time to read this MFT than the smaller MFTs of smaller partitions.

PC BIOS partition types

This section describes partitions as used in MS-DOS, Microsoft Windows and Linux on PC compatible computer systems; for examples of partitioning schemes used in other operating systems, see Slice (disk) and BSD disklabel.

A PC hard disk can contain either as many as four primary partitions, or 1-3 primaries and a single extended partition. Each of these partitions are described by a 16-byte entry in the Partition Table which is located in the Master Boot Record.

The "type" of a partition is identified by a 1-byte code found in its partition table entry. Some of these codes (such as 0x05 and 0x0F) may be used to indicate the presence of an extended partition, but most are used by operating systems that examine partition tables to decide if a partition contains a file system they can mount/access for reading or writing data.

Once a specific partition's type has been identified, additional information about its purpose and probable contents may be found (see: List of partition identifiers for PCs as one such resource). For example, some type codes are used to hide a partition's contents from various operating systems. However, if an OS or some partitioning tool has been programmed to also examine the boot sectors of any partition, then its file system may no longer remain hidden. (Note: There are no officially assigned partition types; thus, more than one kind of file system may lay claim to the same code value.)

Primary (or logical)

A primary (or logical) partition contains one file system. In MS-DOS and earlier versions of Microsoft Windows systems, the first partition (C:) must be a "primary partition". Other operating systems may not share this limitation; however, this can depend on other factors, such as a PC's BIOS.

The "partition type" code for a primary or logical partition can either correspond to a file system contained within (e.g. 0x07 means either an NTFS or an OS/2 HPFS file system) or indicate the partition has a special use (e.g. code 0xBC may mean an Acronis Secure Zone and code 0x82 usually indicates a Linux swap partition). The FAT16 and FAT32 file systems have made use of quite a number of partition type codes over time due to the limits of various DOS and Windows OS versions. Though a Linux operating system may recognize a number of different file systems (ext2, ext3, reiserfs, etc.), they have all consistently used the same partition type code: 0x83 (Linux native).

Extended

An extended partition is secondary to the primary partition(s). A hard disk may contain only one extended partition; which can then be sub-divided into logical drives, each of which is (under DOS and Windows) assigned additional drive letters.

For example, under either DOS or Windows, a hard disk with one primary partition and one extended partition, the latter containing two logical drives, would typically be assigned the three drive letters: C:, D: and E: (in that order).See extended boot record for information on the structure of an extended partition.