6. Disk geometry, partitions and `overlap'

If you have several operating systems on your disks, then each uses one or more disk partitions. A disagreement on where these partitions are may have catastrophic consequences.

The MBR contains a partition table describing where the (primary) partitions are. There are 4 table entries, for 4 primary partitions, and each looks like

struct partition {
        char active;    /* 0x80: bootable, 0: not bootable */
        char begin[3];  /* CHS for first sector */
        char type;
        char end[3];    /* CHS for last sector */
        int start;      /* 32 bit sector number (counting from 0) */
        int length;     /* 32 bit number of sectors */
};

This information is redundant: the location of a partition is given both by the 24-bit begin and end fields, and by the 32-bit start and length fields.

Linux only uses the start and length fields, and can therefore handle partitions of not more than 2^32 sectors, that is, partitions of at most 2 TiB. That is thirty times larger than the disks available today, so maybe it will be enough for the next six years or so. (So, partitions can be very large, but there is a serious restriction in that a file in an ext2 filesystem on hardware with 32-bit integers cannot be larger than 2 GiB.)

DOS uses the begin and end fields, and uses the BIOS INT13 call to access the disk, and can therefore only handle disks of not more than 8.4 GB, even with a translating BIOS. (Partitions cannot be larger than 2.1 GB because of restrictions of the FAT16 file system.) The same holds for Windows 3.11 and WfWG and Windows NT 3.*.

Windows 95 has support for the Extended INT13 interface, and uses special partition types (c, e, f instead of b, 6, 5) to indicate that a partition should be accessed in this way. When these partition types are used, the begin and end fields contain dummy information (1023/255/63). Windows 95 OSR2 introduces the FAT32 file system (partition type b or c), that allows partitions of size at most 2 TiB.

What is this nonsense you get from fdisk about `overlapping' partitions, when in fact nothing is wrong? Well - there is something `wrong': if you look at the begin and end fields of such partitions, as DOS does, they overlap. (And that cannot be corrected, because these fields cannot store cylinder numbers above 1024 - there will always be `overlap' as soon as you have more than 1024 cylinders.) However, if you look at the start and length fields, as Linux does, and as Windows 95 does in the case of partitions with partition type c, e or f, then all is well. So, ignore these warnings when cfdisk is satisfied and you have a Linux-only disk. Be careful when the disk is shared with DOS. Use the commands cfdisk -Ps /dev/hdx and cfdisk -Pt /dev/hdx to look at the partition table of /dev/hdx.