The Linux IDE driver gets the geometry and capacity of a disk
(and lots of other stuff) by using an ATA IDENTIFY request.
Until recently the driver would not believe the returned value
of lba_capacity if it was more than 10% larger than the capacity
computed by C*H*S. However, by industry agreement
large IDE disks (with more than 16514064 sectors)
return C=16383, H=16, S=63, for a total of 16514064 sectors (7.8 GB)
independent of their actual size, but give their actual size in
lba_capacity.
Recent Linux kernels (2.0.34, 2.1.90) know about this
and do the right thing. If you have an older Linux kernel and do
not want to upgrade, and this kernel only sees 8 GiB of a much larger disk,
then try changing the routine lba_capacity_is_ok in
/usr/src/linux/drivers/block/ide.c into something like
static int lba_capacity_is_ok (struct hd_driveid *id) {
id->cyls = id->lba_capacity / (id->heads * id->sectors);
return 1;
}
As just mentioned, large disks return the geometry C=16383, H=16, S=63 independent of the actual size, while the actual size is returned in the value of LBAcapacity. Some BIOSes do not recognize this, and translate this 16383/16/63 into something with fewer cylinders and more heads, for example 1024/255/63 or 1027/255/63. So, the kernel must not only recognize the single geometry 16383/16/63, but also all BIOS-mangled versions of it. Since 2.2.2 this is done correctly (by taking the BIOS idea of H and S, and computing C = capacity/(H*S)). Usually this problem is solved by setting the disk to Normal in the BIOS setup (or, even better, to None, not mentioning it at all to the BIOS). If that is impossible because you have to boot from it or use it also with DOS/Windows, and upgrading to 2.2.2 or later is not an option, use kernel boot parameters.
If a BIOS reports 16320/16/63, then this is usually done in order to get 1024/255/63 after translation.
Many disks have jumpers that allow you to choose between a 15-head an a 16-head geometry. The default settings will give you a 16-head disk. Sometimes both geometries address the same number of sectors, sometimes the 15-head version is smaller. There may be a good reason for this setup: Petri Kaukasoina writes: `A 10.1 Gig IBM Deskstar 16 GP (model IBM-DTTA-351010) was jumpered for 16 heads as default but this old PC (with AMI BIOS) didn't boot and I had to jumper it for 15 heads. hdparm -i tells RawCHS=16383/15/63 and LBAsects=19807200. I use 20960/15/63 to get the full capacity.' For the jumper settings, see http://www.storage.ibm.com/techsup/hddtech/hddtech.htm.
Many disks have jumpers that allow you to make the disk appear smaller than it is. A silly thing to do, and probably no Linux user ever wants to use this, but some BIOSes crash on big disks. The usual solution is to keep the disk entirely out of the BIOS setup. But this may be feasible only if the disk is not your boot disk.
The first serious limit was the 4096 cylinder limit (that is, with 16 heads and 63 sectors/track, 2.11 GB). For example, a Fujitsu MPB3032ATU 3.24 GB disk has default geometry 6704/15/63, but can be jumpered to appear as 4092/16/63, and then reports LBAcapacity 4124736 sectors, so that the operating system cannot guess that it is larger in reality. In such a case (with a BIOS that crashes if it hears how big the disk is in reality, so that the jumper is required) one needs boot parameters to tell Linux about the size of the disk.
That is unfortunate. Most disks can be jumpered so as to appear as a 2 GB disk and then report a clipped geometry like 4092/16/63 or 4096/16/63, but still report full LBAcapacity. Such disks will work well, and use full capacity under Linux, regardless of jumper settings.
A more recent limit is the 33.8 GB limit. Linux kernels older than 2.3.21 need a patch to be able to cope with IDE disks larger than this. Some disks larger than this limit can be jumpered to appear as a 33.8 GB disk. For example, the IBM Deskstar 37.5 GB (DPTA-353750) can be jumpered to appear as a 33.8 GB disk, and then reports geometry 16383/16/63 like any big disk, but LBAcapacity 66055248 (corresponding to 65531/16/63, or 4111/255/63)). These, when jumpered for 33.8 GB, again require boot parameters for full capacity under Linux. See also the BIOS 33.8 GB limit.