B.3. The Master Boot Record
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B.3. The Master Boot Record

The Master Boot Record (MBR) is located in the first sector of the primary disk drive. The MBR may be up to 446B of code, and partition tables may be up to 64B of data. When you add in another 2B to record a Boot Signature you have 512B, which up until recently happened to be the typical size of one sector [60] [61] . This first sector is referred to by a special name, the boot sector.

Table B.1. Master Boot Record Contents

Size (in bytes)PercentPurpose
440B86%Bootable Code (such as GRUB [a]/LILO [b])
004B0.8%Disk signature
002B0.4%Nulls
064B13%Partition Table
002B0.4%MBR Signature

[a] The Grand Unified Boot Loader (GRUB): http://www.gnu.org/software/grub/


In the old days a disk cylinder (or track) was typically 63 sectors. This would represent one concentric ring of storage on a physical disk. Some people believe that early operating systems (notably MS-DOS) enforced requirements which dictated that partitions begin on cylinder boundaries, or that the OS needed to begin and end on a cylinder boundary. Jonathan de Boyne Pollard (JDBP) disputes that claim [62] , saying:

 

It is often believed that disc partitions have to be aligned to cylinder or track boundaries. This is not in fact true and never really has been. There are alignment considerations for disc partitions, but they have nothing to do with cylinders, and they aren't mandatory. Operating systems will still work with misaligned partitions, just more slowly for some (not all) disc unit models.

The idea that disc partitions have to aligned to cylinder boundaries is nonsense on its face. Millions of people have had discs where the first primary partition began on track zero, sector one, head one with no ill effect whatsoever on operating systems from MS-DOS through Windows NT to OS/2. That was, after all, the default that fdisk/Disk Manager on those operating systems used for almost two decades. At best, the purported alignment requirement would have been a track alignment, with all partitions starting at sector one (Sectors are numbered from one, remember.) of any given track.

But this is not true, either. No version of any operating system has actually required this. Even MS-DOS was quite happy to have disc partitions starting at sectors other than 1. The only things that have required this have been disc partitioning utilities. There's been a bit of circular logic about this. The disc partitioning utilities enforced the requirement because their authors thought that it was a requirement, but people only thought that it was a requirement because fdisk and the like enforced it. It was what the partitioning utility programs enforced — so the logic went — so it must have been a restriction. In fact it never was, and no operating system itself has any trouble with this.

 
 --Jonathan de Boyne Pollard

What we can take away from JDBP here is this: Operating systems, not even MS-DOS, require partition's to begin (or end) on cylinder or track boundaries.

The very idea that partitions have such restrictions is a complete falsehood. A story passed down from hacker generation to generation, accepted as canon and never questioned.

JDBP goes on to also discuss the 4KiB alignment rule:

There is a disc partition alignment rule that does reflect the actual hardware. It is the rule that partitions be aligned to 4KiB boundaries. This rule only makes sense for some hard disc models, however.

In some hard disc models, the internal sector size has been increased from 0.5KiB to 4KiB. At the I/O command level, as system softwares access the disc, the sector size is still 0.5KiB, however. Such discs are known as “512 byte emulation” discs […]

What happens on such “512e” discs is that whenever the operating system or the firmware reads a 0.5KiB sector, the disc unit itself is actually reading a whole 4KiB and handing the firmware/operating system the appropriate one-eighth; and whenever the firmware/operating system writes a 0.5KiB sector, the disc unit is actually reading a whole 4KiB sector, modifying one eighth, and writing the whole 4KiB back again.

[…]

So it's simply necessary to ensure that those eight 0.5KiB sectors are contiguous and aligned to an actual 4KiB sector on the disc. The “natural” I/O boundaries used by the operating system must align with the internal, hidden, 4KiB boundaries of the physical disc. The eight 0.5KiB sectors in the I/O command must not span two or more 4KiB physical sectors; but must be exactly one 4KiB sector, and in the right order within that sector.

What we should first observe from this second quote is that there is a rule regarding sector alignment. But that rule has nothing to do with operating system requirements. Furthermore, this is only a rule and we are not obligated to follow it. Failure to follow the rule simply results in degraded I/O performance.

I recommend reading the entire page for a complete overview of these topics. JDBP does an excellent job separating the fact from fiction and explains how you can achieve correct 4KiB alignment, or realignment if you need to fix an existing system.

[Note]Note

For more information on the “native” 4KiB disk drive topic I recommend reviewing footnote [60].


[60] Seagate.com - Transition to Advanced Format 4K Sector Hard Drives: http://www.seagate.com/tech-insights/advanced-format-4k-sector-hard-drives-master-ti/

[61] Pixel Beat - Details of GRUB on the PC: http://www.pixelbeat.org/docs/disk/

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