On Wed, Sep 03, 2014 at 01:27:00PM +0800, shhuiw wrote: > 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.
The 'cow' format is an old format that is rarely used. It's not a good
example.
qcow2 is actively developed and performs better. Unfortunately it is a
lot more complex.
> 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
$ sudo yum install dwarves
$ pahole block/cow.o
...
struct cow_header_v2 {
uint32_t magic; /* 0 4 */
uint32_t version; /* 4 4 */
char backing_file[1024]; /* 8 1024 */
/* --- cacheline 16 boundary (1024 bytes) was 8 bytes ago --- */
int32_t mtime; /* 1032 4 */
/* XXX 4 bytes hole, try to pack */
uint64_t size; /* 1040 8 */
uint32_t sectorsize; /* 1048 4 */
/* size: 1056, cachelines: 17, members: 6 */
/* sum members: 1048, holes: 1, sum holes: 4 */
/* padding: 4 */
/* last cacheline: 32 bytes */
};
Ooops, the compiler has inserted padding into the struct because the
size field was not aligned.
We can't change this because QEMU must stay backwards compatible.
Stefan
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