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?
