Re: [Qemu-devel] External COW format for raw images

[Frediano Ziglio]

2011/7/19 Robert Wang <address@hidden>:
> 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.
>

I don't understand if you mean just a way to track clusters/sectors
changed or a new way to implement snapshotting, something that writing
data just store original to cow like like:


normal backfile

is allocated on image
  write to image
else
  allocate on image
  copy from backing to image
  write to image (patch before previous write)

cow-raw (inverse backfile)

is allocated on image
  write to backing
else
  allocate on image
  copy from backing to image
  write to backing


that is


is not allocated on image
  allocate on image
  copy from backing to image
is normal backing
  write to image
else
  write to backing


Frediano