[Qemu-devel] fdisk give wrong number for losetup

[Qemu-devel] fdisk give wrong number for losetup

[octane indice]

Hello

I have some trouble using losetup to mount partitions
from the host.

root@zipslack:/usr/src/qemu-test# fdisk -l zipslack.img
Vous devez initialiser cylindres.
Vous pouvez faire cela à partir du menu des fonctions additionnelles.

Disque zipslack.img: 0 Mo, 0 octets
16 têtes, 63 secteurs/piste, 0 cylindres
Unités = cylindres de 1008 * 512 = 516096 octets

 Périphérique Amorce    Début         Fin      Blocs    Id  Système
zipslack.img1               1          49       24664+  82  Linux swap
zipslack.img2              50         253      102816   83  Linux
zipslack.img3             254         609      179424   83  Linux
root@zipslack:/usr/src/qemu-test#losetup -f
/dev/loop1
root@zipslack:/usr/src/qemu-test# losetup -o $((50*512)) /dev/loop1 zipslack.img
root@zipslack:/usr/src/qemu-test# mount -t ext2 /dev/loop1 /mnt/hd
mount: wrong fs type, bad option, bad superblock on /dev/loop1,
       missing codepage or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so
root@zipslack:/usr/src/qemu-test# sfdisk zipslack.img
AVERTISSEMENT: zipslack.img n'est pas un périphérique fonctionnant par blocs
Disque zipslack.img: ne peut obtenir la géométrie

Disque zipslack.img: 0 cylindres, 0 têtes, 0 secteurs/piste
Vieille situation:
AVERTISSEMENT: la table de partitions semble avoir été créée
  pour C/H/S=*/16/63 (au lieu de 0/0/0).
Pour ce rapport, la géométrie suivante sera assumée.
Unités= cylindres de 516096 octets, blocs de 1024 octets, décompte à partir de 0

   Périph Amor Début     Fin   #cyls    #blocs    Id  Système
zipslack.img1          0+     48      49-     24664+  82  Linux swap
zipslack.img2         49     252     204     102816   83  Linux
zipslack.img3        253     608     356     179424   83  Linux
zipslack.img4          0       -       0          0    0  Vide
L'entrée utiliser un des formats suivants; les champs absents prennent
la valeur par défaut:
<début> <taille> <type [E,S,L,X,hex]> <amorçable [-,*]> <c,h,s> <c,h,s>
Habituellement vous n'avez besoin que de spécifier:
<début> et <taille> (et parfois <type>).

zipslack.img1 :
root@zipslack:/usr/src/qemu-test#
root@zipslack:/usr/src/qemu-test# losetup -f
/dev/loop2
root@zipslack:/usr/src/qemu-test# losetup -o $((49*516096)) /dev/loop2
zipslack.img
root@zipslack:/usr/src/qemu-test# mount -t ext2 /dev/loop2 /mnt/hd/
root@zipslack:/usr/src/qemu-test# ls /mnt/hd/
bin/   cdrom/  etc/     home/     lib/         mnt/   root/  tmp/  var/
boot/  dev/    faq.txt  install/  lost+found/  proc/  sbin/  usr/  vmlinuz
root@zipslack:/usr/src/qemu-test#

And now, my questions: why this?

fdisk doesn't give the good values?
what's this number given by sfdisk: 516096 ?

thanks

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

[Jim C. Brown]

On Thu, Jul 07, 2005 at 12:36:08PM +0200, octane indice wrote:
> Hello
> 
> 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.

> 
> 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.)

> 
> ---------------------------------------------
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> 
> 
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> Qemu-devel@nongnu.org
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> 

-- 
Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.

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

[Henrik Nordstrom]

On Thu, 7 Jul 2005, octane indice wrote:

> Hello
>
> I have some trouble using losetup to mount partitions
> from the host.
>
> root@zipslack:/usr/src/qemu-test# fdisk -l zipslack.img
> Vous devez initialiser cylindres.
> Vous pouvez faire cela à partir du menu des fonctions additionnelles.
>
> Disque zipslack.img: 0 Mo, 0 octets
> 16 têtes, 63 secteurs/piste, 0 cylindres
> Unités = cylindres de 1008 * 512 = 516096 octets
>
> Périphérique Amorce    Début         Fin      Blocs    Id  Système
> zipslack.img1               1          49       24664+  82  Linux swap
> zipslack.img2              50         253      102816   83  Linux
> zipslack.img3             254         609      179424   83  Linux

fdisk -l -u zipslack.img

or

sfdisk -d zipslack.img

gives a much better view of the partition table.. (blocks, not cylinders)

Regards
Henrik

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

[octane]

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.

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

[Henrik Nordstrom]

On Sun, 10 Jul 2005, octane wrote:

> 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

The offset if 67*516096, as this partition table is printed in cylinders, 
not sectors.

When calculating partition offsets it is always best to look at the 
partition table in sectors, not cylinders, especially considering that 
partitions does not need to start exactly on a cylinder boundary. For 
example the first partition doesn't start at cylinder 1, it starts at 
cylinder 1 head 2, or sector 64. Both fdisk and sfdisk supports viewing 
the sector based partition values, maintaining sanity.

>> 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.)

Nearly nobody except for DOS (and the parts of BIOS used by DOS) cares 
about geometry..

> 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))

Correct, from the cylinders/heads/sectors geometry. If you look at the 
swap partition then things is more interesting.. (also why sfdisk printed 
a + there, and probably what the + in the blocks column of fdisk 
refers to...)

Regards
Henrik