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.
Many IBM 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.' The geometry 16383/15/63 is not yet recognized by the kernel, so explicit boot parameters are required here. For the jumper settings, see http://www.storage.ibm.com/techsup/hddtech/hddtech.htm.
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