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## 12. The Linux 65535 cylinder limit

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).)

## 12.1 IDE problems with 34+ GB disks

(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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