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The HDIO_GETGEO ioctl returns the number of cylinders in a short.
This means that if you have more than 65535 cylinders, the number is
truncated, and (for a typical SCSI setup with 1 MiB cylinders)
a 80 GiB disk may appear as a 16 GiB one.
Once one recognizes what the problem is, it is easily avoided.
Use fdisk 2.10i or newer.
(The programming convention is to use the BLKGETSIZE ioctl
to get total size, and HDIO_GETGEO to get number of heads and
sectors/track, and, if needed, get C by C = size/(H*S).)
(Below a discussion of Linux kernel problems. BIOS problems
and jumpers that clip capacity were discussed
above.)
Drives larger than 33.8 GB will not work with kernels older than
2.0.39 / 2.2.14 / 2.3.21.
The details are as follows.
Suppose you bought a new IBM-DPTA-373420 disk with a capacity
of 66835440 sectors (34.2 GB). Pre-2.3.21 kernels will tell you
that the size is 769*16*63 = 775152 sectors (0.4 GB), which
is a bit disappointing. And giving command line parameters
hdc=4160,255,63 doesn't help at all - these are just ignored.
What happens? The routine idedisk_setup()
retrieves the geometry reported by the disk (which is
16383/16/63) and overwrites what the user specified on
the command line, so that the user data is used only
for the BIOS geometry. The routine current_capacity()
or idedisk_capacity() recomputes the cylinder number as
66835440/(16*63)=66305, but since this is stored in a short,
it becomes 769. Since lba_capacity_is_ok() destroyed id->cyls,
every following call to it will return false, so that the
disk capacity becomes 769*16*63.
For several kernels a patch is available.
A patch for 2.0.38 can be found at
ftp.kernel.org.
A patch for 2.2.12 can be found at
www.uwsg.indiana.edu
(some editing may be required to get rid of the html markup).
The 2.2.14 kernels do support these disks.
In the 2.3.* kernel series, there is support for these disks
since 2.3.21.
One can also `solve' the problem in hardware by
using a jumper to clip the size to 33.8 GB.
In many cases a
BIOS upgrade will be
required if one wants to boot from the disk.
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