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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 A patch for 2.2.12 can be found at (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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