Large Disk HOWTO

8. Kernel disk translation for IDE disks

If the Linux kernel detects the presence of some disk manager on an IDE disk, it will try to remap the disk in the same way this disk manager would have done, so that Linux sees the same disk partitioning as for example DOS with OnTrack or EZ-Drive. However, NO remapping is done when a geometry was specified on the command line - so a `hd=cyls,heads,secs' command line option might well kill compatibility with a disk manager.

If you are hit by this, and know someone who can compile a new kernel for you, find the file linux/drivers/block/ide.c and remove in the routine ide_xlate_1024() the test if (drive->forced_geom) { ...; return 0; }.

The remapping is done by trying 4, 8, 16, 32, 64, 128, 255 heads (keeping H*C constant) until either C <= 1024 or H = 255.

The details are as follows - subsection headers are the strings appearing in the corresponding boot messages. Here and everywhere else in this text partition types are given in hexadecimal.

8.1 EZD

EZ-Drive is detected by the fact that the first primary partition has type 55. The geometry is remapped as described above, and the partition table from sector 0 is discarded - instead the partition table is read from sector 1. Disk block numbers are not changed, but writes to sector 0 are redirected to sector 1. This behaviour can be changed by recompiling the kernel with #define FAKE_FDISK_FOR_EZDRIVE 0 in ide.c.

8.2 DM6:DDO

OnTrack DiskManager (on the first disk) is detected by the fact that the first primary partition has type 54. The geometry is remapped as described above and the entire disk is shifted by 63 sectors (so that the old sector 63 becomes sector 0). Afterwards a new MBR (with partition table) is read from the new sector 0. Of course this shift is to make room for the DDO - that is why there is no shift on other disks.

8.3 DM6:AUX

OnTrack DiskManager (on other disks) is detected by the fact that the first primary partition has type 51 or 53. The geometry is remapped as described above.

8.4 DM6:MBR

An older version of OnTrack DiskManager is detected not by partition type, but by signature. (Test whether the offset found in bytes 2 and 3 of the MBR is not more than 430, and the short found at this offset equals 0x55AA, and is followed by an odd byte.) Again the geometry is remapped as above.

8.5 PTBL

Finally, there is a test that tries to deduce a translation from the start and end values of the primary partitions: If some partition has start and end sector number 1 and 63, respectively, and end heads 31, 63, 127 or 254, then, since it is customary to end partitions on a cylinder boundary, and since moreover the IDE interface uses at most 16 heads, it is conjectured that a BIOS translation is active, and the geometry is remapped to use 32, 64, 128 or 255 heads, respectively. However, no remapping is done when the current idea of the geometry already has 63 sectors per track and at least as many heads (since this probably means that a remapping was done already).

8.6 Getting rid of a disk manager

When Linux detects OnTrack Disk Manager, it will shift all disk accesses by 63 sectors. Similarly, when Linux detects EZ-Drive, it shifts all accesses of sector 0 to sector 1. This means that it may be difficult to get rid of these disk managers. Most disk managers have an uninstall option, but if you need to remove some disk manager an approach that often works is to give an explicit disk geometry on the command line. Now Linux skips the ide_xlate_1024() routine, and one can wipe out the partition table with disk manager (and probably lose access to all disk data) with the command

        dd if=/dev/zero of=/dev/hdx bs=512 count=1

8.7 Since 2.5.70: boot parameters

In 2.5.70 the automatic disk manager support was removed. Instead, two boot options were added: "hda=remap" to do the EZ-Drive remapping of sector 0 to sector 1, and "hda=remap63" to do the OnTrack Disk Manager shift over 63 sectors.

This also means that it no longer is a problem to get rid of a disk manager.