71 lines
3.2 KiB
HTML
71 lines
3.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>Large Disk HOWTO: The Linux 65535 cylinder limit</TITLE>
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<LINK HREF="Large-Disk-HOWTO-13.html" REL=next>
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<LINK HREF="Large-Disk-HOWTO-11.html" REL=previous>
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<LINK HREF="Large-Disk-HOWTO.html#toc12" REL=contents>
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<A HREF="Large-Disk-HOWTO-13.html">Next</A>
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<A HREF="Large-Disk-HOWTO.html#toc12">Contents</A>
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<HR>
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<H2><A NAME="s12">12. The Linux 65535 cylinder limit</A></H2>
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<P>The <CODE>HDIO_GETGEO</CODE> ioctl returns the number of cylinders in a short.
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This means that if you have more than 65535 cylinders, the number is
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truncated, and (for a typical SCSI setup with 1 MiB cylinders)
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a 80 GiB disk may appear as a 16 GiB one.
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Once one recognizes what the problem is, it is easily avoided.
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Use fdisk 2.10i or newer.
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<P>(The programming convention is to use the <CODE>BLKGETSIZE</CODE> ioctl
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to get total size, and <CODE>HDIO_GETGEO</CODE> to get number of heads and
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sectors/track, and, if needed, get C by C = size/(H*S).)
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<H2><A NAME="verylarge"></A> <A NAME="ss12.1">12.1 IDE problems with 34+ GB disks</A>
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</H2>
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<P>(Below a discussion of Linux kernel problems. BIOS problems
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and jumpers that clip capacity were discussed
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<A HREF="Large-Disk-HOWTO-11.html#jumperbig">above</A>.)
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<P>Drives larger than 33.8 GB will not work with kernels older than
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2.0.39 / 2.2.14 / 2.3.21.
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The details are as follows.
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Suppose you bought a new IBM-DPTA-373420 disk with a capacity
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of 66835440 sectors (34.2 GB). Pre-2.3.21 kernels will tell you
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that the size is 769*16*63 = 775152 sectors (0.4 GB), which
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is a bit disappointing. And giving command line parameters
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hdc=4160,255,63 doesn't help at all - these are just ignored.
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What happens? The routine idedisk_setup()
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retrieves the geometry reported by the disk (which is
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16383/16/63) and overwrites what the user specified on
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the command line, so that the user data is used only
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for the BIOS geometry. The routine current_capacity()
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or idedisk_capacity() recomputes the cylinder number as
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66835440/(16*63)=66305, but since this is stored in a short,
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it becomes 769. Since lba_capacity_is_ok() destroyed id->cyls,
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every following call to it will return false, so that the
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disk capacity becomes 769*16*63.
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For several kernels a patch is available.
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A patch for 2.0.38 can be found at
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<A HREF="ftp://ftp.us.kernel.org/pub/linux/kernel/people/aeb/">ftp.kernel.org</A>.
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A patch for 2.2.12 can be found at
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<A HREF="http://www.uwsg.indiana.edu/hypermail/linux/kernel/9910.2/0636.html">www.uwsg.indiana.edu</A>
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(some editing may be required to get rid of the html markup).
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The 2.2.14 kernels do support these disks.
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In the 2.3.* kernel series, there is support for these disks
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since 2.3.21.
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One can also `solve' the problem in hardware by
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<A HREF="Large-Disk-HOWTO-11.html#jumperbig">using a jumper</A> to clip the size to 33.8 GB.
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In many cases a
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<A HREF="Large-Disk-HOWTO-4.html#biosupgrades">BIOS upgrade</A> will be
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required if one wants to boot from the disk.
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<P>
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<HR>
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<A HREF="Large-Disk-HOWTO-13.html">Next</A>
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<A HREF="Large-Disk-HOWTO-11.html">Previous</A>
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<A HREF="Large-Disk-HOWTO.html#toc12">Contents</A>
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