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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