[Qemu-devel] Question about cow format with hexdump

[shhuiw]

Hi,

I'm reading the source code of cow.c: 
https://github.com/qemu/qemu/blob/master/block/cow.c
and try to understand the format better.

I created a cow format imagefile and can run 'qume-img info' to query the 
header info
-------------------------------------------------------------------------------
-bash-4.1$ qemu-img create -f cow dummy 1M
Formatting 'test/dummy', fmt=cow size=1048576 
-bash-4.1$ qemu-img info dummy 
image: dummy
file format: cow
virtual size: 1.0M (1048576 bytes)
disk size: 4.0K


But when I used hexdump to dis the header part, I cannot find all info recorded:
(compared the define of struct cow_header_v2 and cow_create())
------------------------------------------------------------------------------
1) recognize the magic and version info:
-bash-4.1$ hexdump -C dummy -n 8
00000000  4f 4f 4f 4d 00 00 00 02                           |OOOM....|
00000008

2) backing_file and mtime fields are 0s:                                        
    # I think the "dummy" should be recorded
-bash-4.1$ hexdump -C dummy -n 1032
00000000  4f 4f 4f 4d 00 00 00 02  00 00 00 00 00 00 00 00  |OOOM............|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000400  00 00 00 00 00 00 00 00                           |........|
00000408
-bash-4.1$ hexdump -C dummy -n 1036
00000000  4f 4f 4f 4d 00 00 00 02  00 00 00 00 00 00 00 00  |OOOM............|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000400  00 00 00 00 00 00 00 00  00 00 00 00              |............|
0000040c

3) size field is 0s:
-bash-4.1$ hexdump -C dummy -n 1044                                             
   # size should be 1M
00000000  4f 4f 4f 4d 00 00 00 02  00 00 00 00 00 00 00 00  |OOOM............|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000410  00 00 00 00                                       |....|
00000414

4) sectorsize is 512:
-bash-4.1$ hexdump -C dummy -n 1048
00000000  4f 4f 4f 4d 00 00 00 02  00 00 00 00 00 00 00 00  |OOOM............|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000410  00 00 00 00 00 10 00 00                           |........|
00000418


Can anyone help to explain this? Or how to debug further?

--

Regards,
shhuiw