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

[Anthony Liguori]

On 07/19/2011 04:25 AM, Robert Wang wrote:
> 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;
> };

I'd suggest that doing an image format is the wrong approach here.  Why
not just have a image format where you can pass it the location of a
bitmap?  That let's you compose arbitrarily complex backing file chains
and avoids the introduce of a new bitmap.

The bitmap format is also useful for implementing things like dirty
tracking.

Regards,

Anthony Liguori

>       * 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.
> 
>