Re: [Qemu-devel] fdisk give wrong number for losetup

[octane <octane@alinto.com>]

Jim C. Brown a écrit :

>>I have some trouble using losetup to mount partitions
>>from the host.
>>    
>>
>
>I never realized how difficult it was to interpret error messages written in
>a foreign language.
>
>  
>
Ok, sorry, in plain english messages are:
root@zipslack:/usr/src/qemu-test# fdisk -l zipslack.img
You must set cylinders.
You can do this from the extra functions menu.

Disk zipslack.img: 0 MB, 0 bytes
16 heads, 63 sectors/track, 0 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

       Device Boot      Start         End      Blocks   Id  System
zipslack.img1               1          66       33232+  82  Linux swap
zipslack.img2              67         609      273672   83  Linux
root@zipslack:/usr/src/qemu-test#

and mounting the file with an offset of $((67*512)) doesn't work

root@zipslack:/usr/src/qemu-test# sfdisk zipslack.img
Warning: zipslack.img is not a block device
Disk zipslack.img: cannot get geometry

Disk zipslack.img: 0 cylinders, 0 heads, 0 sectors/track
Old situation:
Warning: The partition table looks like it was made
  for C/H/S=*/16/63 (instead of 0/0/0).
For this listing I'll assume that geometry.
Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
zipslack.img1          0+     65      66-     33232+  82  Linux swap
zipslack.img2         66     608     543     273672   83  Linux
zipslack.img3          0       -       0          0    0  Empty
zipslack.img4          0       -       0          0    0  Empty
Input in the following format; absent fields get a default value.
<start> <size> <type [E,S,L,X,hex]> <bootable [-,*]> <c,h,s> <c,h,s>
Usually you only need to specify <start> and <size> (and perhaps <type>).

zipslack.img1 :

And mounting with $((66*516096)) is ok.


>>And now, my questions: why this?
>>    
>>
>
>Because you were passing the wrong value as the offset. You have to multipy
>the cylinder with the number of heads and number of sectors per head and such.
>  
>
>>what's this number given by sfdisk: 516096 ?
>>

>>Apparently, head*sectors per head*bytes per sector ... my fdisk doesn't show
>>that number though.
>>
>>    
>>
>>fdisk doesn't give the good values?
>>
>>thanks
>>    
>>
>
>It does, but they are hard to interpret unless you understand disk geometries.
>
>I really recommend using lomount or some similar tool to figure out these
>offsets for you. (Incidently, lomount doesn't care about disk geometry - it
>uses the absolute sector values in the partition table.)
>  
>
I receive a mail that says:
fdisk -l zipslack.img is not the same as fdisk- lu zipslack :
root@zipslack:/usr/src/qemu-test# fdisk -lu zipslack.img
You must set cylinders.
You can do this from the extra functions menu.

Disk zipslack.img: 0 MB, 0 bytes
16 heads, 63 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes

       Device Boot      Start         End      Blocks   Id  System
zipslack.img1              63       66527       33232+  82  Linux swap
zipslack.img2           66528      613871      273672   83  Linux

and incidently $((66*516096))=$((66528*512))

thanks.