Script partitioning fresh raw disk images with sfdisk (C/H/S ignored?)

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Script partitioning fresh raw disk images with sfdisk (C/H/S ignored?) – Managing your servers can streamline the performance of your team by allowing them to complete complex tasks faster. Plus, it can enable them to detect problems early on before they get out of hand and compromise your business. As a result, the risk of experiencing operational setbacks is drastically lower.

But the only way to make the most of your server management is to perform it correctly. And to help you do so, this article will share nine tips on improving your server management and fix some problem about partition, disk-image, linux, , .

I’m trying to automate the process of creating raw disk images. I don’t care so much about C/H/S, but that the block size should be a rough standard of 512 bytes. However, I’m having trouble specifying the correct dimensions for partitions so that sfdisk can import them.

I first created a 32MB blank file:

$dd if=/dev/zero of=disk.img bs=1M count=32

Then I partitioned it using cfdisk:

$cfdisk -h 255 -s 63 -c 4 disk.img
  • Verify that cfdisk sees the C/H/S switches (at the top of the screen)
  • Create new partition 32.5MB, end of disk (to leave space for the boot stuff later)
  • Write
  • Quit

Then I output an sfdisk dump:

$sfdisk -H 255 -S 63 -C 4 -d disk.img > disk.parts

and got the following (in disk.parts):

# partition table of disk.img
unit: sectors

disk.img1 : start=     1276, size=    64260, Id=83, bootable
disk.img2 : start=        0, size=        0, Id= 0
disk.img3 : start=        0, size=        0, Id= 0
disk.img4 : start=        0, size=        0, Id= 0

However, when I try to reload that back into the image (as a test), sfdisk seems to at first accept the C/H/S switches when reading the original partition table and then throw them away when it tries to calculate the new partition table:

$sfdisk -H 255 -S 63 -C 4 disk.img < disk.parts

Warning: disk.img is not a block device
Disk disk.img: cannot get geometry

Disk disk.img: 4 cylinders, 255 heads, 63 sectors/track
Old situation:
Warning: The partition table looks like it was made
  for C/H/S=*/21/16 (instead of 4/255/63).
For this listing I'll assume that geometry.
Units = cylinders of 172032 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
disk.img1   *      3+    195-    192-     32130   83  Linux
        start: (c,h,s) expected (3,16,13) found (0,20,17)
        end: (c,h,s) expected (195,0,16) found (4,20,16)
disk.img2          0       -       0          0    0  Empty
disk.img3          0       -       0          0    0  Empty
disk.img4          0       -       0          0    0  Empty
New situation:
Warning: The partition table looks like it was made
  for C/H/S=*/21/16 (instead of 4/255/63).
For this listing I'll assume that geometry.
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
disk.img1   *      1276     65535      64260  83  Linux
        start: (c,h,s) expected (3,16,13) found (0,20,17)
        end: (c,h,s) expected (195,0,16) found (4,20,16)
disk.img2             0         -          0   0  Empty
disk.img3             0         -          0   0  Empty
disk.img4             0         -          0   0  Empty
Warning: partition 1 does not end at a cylinder boundary
end of partition 1 has impossible value for cylinders: 4 (should be in 0-3)

sfdisk: I don't like these partitions - nothing changed.
(If you really want this, use the --force option.)

It appears that these two sections are in conflict:

 Warning: The partition table looks like it was made
  for C/H/S=*/21/16 (instead of 4/255/63).
 For this listing I'll assume that geometry.
 Units = cylinders of 172032 bytes, blocks of 1024 bytes, counting from 0

and

 For this listing I'll assume that geometry.
 Units = sectors of 512 bytes, counting from 0

And it then reinforces it with:

 end of partition 1 has impossible value for cylinders: 4 (should be in 0-3)

I tried -f (force) and it gives exactly the same output. 🙁

Why would sfdisk not process it’s own dump format correctly, particularly when I’m already giving it all the information it needs? Why would it process the C/H/S when reading but not when writing? The C/H/S is not in the file, so why would it say that it looks like it was made for */21/16?

More importantly, how do I get around this so I can generate a partition table in a script?

Solution :

C/H/S is pretty much obsolete and shouldn’t be used. I’m not very familiar with sfdisk specifically, but any recent partitioning tool should let you specify your partition boundaries in term of 512-byte sectors, and for performance reasons they should always be 4k-aligned (so divisible by 8).

The specific issue you seem to have is that sfdisk cannot detect C/H/S on the image file (since it’s not a block device) and end up with bogus values.

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