07-A.2: Configuring Boot Components (continued)

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Boot Source Options

In addition to the normal idea of booting from the system's hard drive, there are other options that are used in various situations.

Boot source	Description
ISO	Most Linux distributions can be downloaded as an ISO image. These are images of an actual Linux system. It is possible to boot directly from an ISO file that resides on a hard disk, or on a USB device. It is also possible to view the file system found on an ISO image using an archiving utility.
Network File System (NFS)	Network File Systems (NFS) can be used as a source of the boot files. This simply means that the boot files are not physically loaded on the machine being booted. The files are accessed across the network. This method must be set up both on the local system and the remote NFS server where the files are located. This boot method makes use of NFS, tftp or other network protocols to retrieve the necessary files.
HTTP/FTP	HTTP Boot combines DHCP, DNS, and HTTP to make it possible to boot and deploy systems over the network. HTTP Boot can be used as a high-performance replacement for PXE. HTTP Boot allows you to boot a server from a URI over HTTP, quickly transferring large files such as the Linux kernel and root file system from servers outside of your local network.
PXE	The Preboot eXecution Environment (PXE, also known as Pre-Execution Environment; sometimes pronounced "pixie") is an environment to boot computers using a network interface independently of data storage devices (like hard disks) or installed operating systems.

What are Sectors

When we discuss booting and MBRs we often mention "sectors." The sector is the minimum storage unit of a hard drive. Most disk partitioning schemes are designed to have files occupy an integral number of sectors regardless of the file's actual size. Files that do not fill a whole sector will have the remainder of their last sector filled with zeroes. In practice, operating systems typically operate on blocks of data, which may span multiple sectors. In the image to the right, 'C' points to a sector. Notice that sectors get bigger the closer they get to the outer edge of the disk (of course we are moving towards solid state devices now - this may become a moot point).

A track is a ring around the disk, like a ring on a bullseye, there are numerous tracks on a disk. A sector is a pie shaped portion of the disk. Every sector has track sectors which is the portion of a track within that sector. A cluster is a group of one or more concurrent track sectors — Figure \(\PageIndex{1}\): Disk structures:

A. Track

B. Geometrical sector

C. Track sector

D. Cluster

("disk sectors" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0)

Master Boot Record

A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives. The MBR holds the information on how the logical partitions, containing file systems, are organized on that medium. The MBR also contains executable code to function as a loader for the installed operating system—usually by passing control over to the loader's second stage. This MBR code is usually referred to as a boot loader.[1]

The organization of the partition table in the MBR limits the maximum addressable storage space of a partitioned disk to 2 TiB (232 × 512 bytes). Approaches to slightly raise this limit assuming 32-bit arithmetic or 4096-byte sectors are not officially supported, as they fatally break compatibility with existing boot loaders and most MBR-compliant operating systems and system tools, and can cause serious data corruption when used outside of narrowly controlled system environments. Therefore, the MBR-based partitioning scheme is in the process of being superseded by the GUID Partition Table (GPT) scheme in new computers. A GPT can coexist with an MBR in order to provide some limited form of backward compatibility for older systems.

The GUID Partition Table

The GUID Partition Table (GPT) is a standard for the layout of partition tables of a physical computer storage device such as a hard disk drive or solid-state drive, using universally unique identifiers, which are also known as globally unique identifiers (GUIDs). Forming a part of the UEFI standard, it is nevertheless also used for some BIOS systems because of the limitations of master boot record (MBR) partition tables, which use 32 bits for logical block addressing (LBA) of traditional 512-byte disk sectors.

All modern personal computer operating systems support GPT. Some, including macOS and Microsoft Windows on the x86 architecture, support booting from GPT partitions only on systems with EFI firmware, but most Linux distributions can boot from GPT partitions on systems with either firmware interface: the legacy BIOS or the modern EFI.

Adapted from:
"Disk sector" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"Master boot record" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"GUID Partition Table" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0