Re: 8 GB limit on cfdisk?

["Mark H. Mabry" <mmabry@avici.com>]

>>>>> "AR" == Richardson,Anthony  <ARichard@stark.cc.oh.us>
>>>>> wrote the following on Wed, 17 Jun 1998 13:52:00 -0400

  AR> 1024 is the maximum number of cylinders that may be stored in a
  AR> partition table entry (10 bits). 1024 is also the maximum
  AR> cylinder that can addressed using the standard BIOS
  AR> routines. (With a tranlating BIOS this limits you to 8 GB, with
  AR> an old BIOS the limit was 504 MB). Fortunately Linux doesn't use
  AR> the BIOS functions to talk to the disk or use the CYL/HEAD/SECT
  AR> addresses in the partition table to locate partitions. You can
  AR> tell Linux that you've got more cylinders with the
  AR> hd=cyls,heads,sects boot option. (cfdisk will ask the kernel for
  AR> the disk geometry. You can also tell cfdisk that you've got more
  AR> than 1024 cylinders with the -c option. See the man page.) Make
  AR> sure the number of heads and sectors match the numbers used by
  AR> the BIOS.

Okay, now we are getting to the meat of the problem.  As you say,
Linux doesn't use the BIOS functions or C/H/S addresses to locate
partitions.  That is why the kernel can see all of my disk at boot.
So why does cfdisk see only 8 GB?  Why doesn't it get its info the
same way the kernel does?  Why does cfdisk have to ask the BIOS for
this info?  

Good to know about the -c option to cfdisk, I missed that one.  

Last night I set the cylinders in cfdisk.  I had two choice on what to
set:  

C/H/S:  16383/16/63  (what my harddisk label says)
  or
C/H/S:  1229/255/63  (the shifted equivalent.  Very close to what
                      cfdisk reported.  What Partition Magic reported.)

I chose the latter so that my Linux partition might line up with my
Win95 partition.  But after I had made all my partitions, written the
partition table, and rebooted (arghh) so that cfdisk could re-read the
partition table, cfdisk would not come up because it said I had a Bad
Partition Table.  Probably due to my last partition extending beyond
the 1023 cylinder boundary.  So I tried fdisk and it let me in fine
and then allowed me to re-set the cylinder number.  I'll try "cfdisk
-c" tonight.

  AR> One note: Since most (all?) boot managers use the BIOS to load
  AR> the OS, you should make sure the kernel stays under the 8 GB
  AR> limit by having your root partition lie entirely under the 8 GB
  AR> limit. (With an old, non- translating BIOS this limit is only
  AR> 504 MB.) How do you know if you have a translating BIOS? If you
  AR> can make a partition greater than 504 MB under DOS, you've got
  AR> one.

Thanks, I did this.

  AR> See the following for detailed info: 
  AR> 1) Large-Disk mini HOWTO 

I read this and found it did not explain this problem very well.  It
seemed geared toward the 504 MB limit.  Only under SCSI does it
reference the 8 GB problem and there it give no solution.  

Mark Mabry
Avici Systems
mmabry@avici.com



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