* [Qemu-devel] Question about cow format with hexdump
@ 2014-09-03 5:27 shhuiw
2014-09-03 10:59 ` Stefan Hajnoczi
2014-09-04 13:28 ` shhuiw
0 siblings, 2 replies; 4+ messages in thread
From: shhuiw @ 2014-09-03 5:27 UTC (permalink / raw)
To: qemu-devel@nongnu.org
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
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] Question about cow format with hexdump
2014-09-03 5:27 [Qemu-devel] Question about cow format with hexdump shhuiw
@ 2014-09-03 10:59 ` Stefan Hajnoczi
2014-09-03 15:35 ` Kevin Wolf
2014-09-04 13:28 ` shhuiw
1 sibling, 1 reply; 4+ messages in thread
From: Stefan Hajnoczi @ 2014-09-03 10:59 UTC (permalink / raw)
To: shhuiw; +Cc: qemu-devel@nongnu.org
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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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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Qemu-devel] Question about cow format with hexdump
2014-09-03 10:59 ` Stefan Hajnoczi
@ 2014-09-03 15:35 ` Kevin Wolf
0 siblings, 0 replies; 4+ messages in thread
From: Kevin Wolf @ 2014-09-03 15:35 UTC (permalink / raw)
To: Stefan Hajnoczi; +Cc: shhuiw, qemu-devel@nongnu.org
[-- Attachment #1: Type: text/plain, Size: 3504 bytes --]
Am 03.09.2014 um 12:59 hat Stefan Hajnoczi geschrieben:
> 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.
We can at least make it explicit like I did for qcow1 in ea54feff.
Kevin
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] Question about cow format with hexdump
2014-09-03 5:27 [Qemu-devel] Question about cow format with hexdump shhuiw
2014-09-03 10:59 ` Stefan Hajnoczi
@ 2014-09-04 13:28 ` shhuiw
1 sibling, 0 replies; 4+ messages in thread
From: shhuiw @ 2014-09-04 13:28 UTC (permalink / raw)
To: shhuiw; +Cc: qemu-devel@nongnu.org
Sorry to trouble you all.
I found the right way: I should use the -o option when I create cow image files.
Then hexdump can give full info.
~> touch testcow # maked up backing_file ahead to get non-0s .mtime
~> qemu-img create -f cow -o backing_file=testcow,size=1M dummy
Formatting 'dummy', fmt=cow size=1048576 backing_file='testcow'
~> hexdump -C dummy
00000000 4f 4f 4f 4d 00 00 00 02 74 65 73 74 63 6f 77 00 |OOOM....testcow.|
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 54 08 66 e8 00 00 00 00 |........T.f.....|
00000410 00 10 00 00 00 00 02 00 00 00 00 00 00 00 00 00 |................|
00000420 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000510 00 00 00 00 00 00 00 00 |........|
00000518
--
Regards,
shhuiw
At 2014-09-03 01:27:00, "shhuiw" <shhuiw@163.com> wrote:
>
>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
^ permalink raw reply [flat|nested] 4+ messages in thread
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2014-09-03 5:27 [Qemu-devel] Question about cow format with hexdump shhuiw
2014-09-03 10:59 ` Stefan Hajnoczi
2014-09-03 15:35 ` Kevin Wolf
2014-09-04 13:28 ` shhuiw
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