[Qemu-devel] External COW format for raw images

[Robert Wang]

As you known, raw image is very popular,but the raw image format does
NOT support Copy-On-Write,a raw image file can NOT be used as a copy
destination, then image streaming/Live Block Copy will NOT work.

To fix this, we need to add a new block driver raw-cow to QEMU. If
finished, we can use qemu-img like this:
qemu-img create -f raw-cow -o backing_file=ubuntu.img,raw_file=my_vm.img
my_vm.raw-cow

1) ubuntu.img is the backing file, my_vm.img is a raw file,
my_vm.raw-cow stores a COW bitmap related to my_vm.img.

2) If the entire COW bitmap is set to dirty flag then we can get all
information from my_vm.img and can ignore ubuntu.img and my_vm.raw-cow
from now.

To implement this, I think I can follow these steps:
1) Add a new member to BlockDriverState struct:
char raw_file[1024];
This member will track raw_file parameter related to raw-cow file from
command line.

2)      * Create a new file block/raw-cow.c. It will be much more like the
mixture of block/cow.c and block/raw.c.

So I will change some functions in cow.c and raw.c to none-static, then
raw-cow.c can re-use them. When read operation occurs, determine whether
dirty flag in raw-cow image is set. If true, read directly from the raw
file. After write operation, set related dirty flag in raw-cow image.
And other functions might also be modified.

        * Of course, format_name member of BlockDriver struct will be "raw-cow".
And in order to keep relationship with raw file( like my_vm.img) ,
raw_cow_header struct should be
struct raw_cow_header {
uint32_t magic;
uint32_t version;
char backing_file[1024];
char raw_file[1024];/* added*/
int32_t mtime;
uint64_t size;
uint32_t sectorsize;
};
        * Struct raw_cow_create_options should be one member plus based on
cow_create_options:
{
.name = BLOCK_OPT_RAW_FILE,
.type = OPT_STRING,
.help = "Raw file name"
},

3) Add bdrv_get_raw_filename in img_info function of qemu-img.c. In
bdrv_get_raw_filename, if the format of the image file is "raw-cow",
print the related raw file.

Do you think my approach is right?
Thank you.