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.