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

[Robert Wang <wdongxu@linux.vnet.ibm.com>]

On Tue, 2011-07-19 at 16:39 +0200, Frediano Ziglio wrote:
> 2011/7/19 Robert Wang <wdongxu@linux.vnet.ibm.com>:
> > 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
> 
Frediano, it mainly is to track clusters/sectors changes, not a new way
to implement snapshotting.