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

[Stefan Hajnoczi]

2011/7/19 Anthony Liguori <address@hidden>:
> 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.

Are you describing something like -drive
file=bitmap:raw.img:backing.img:dirty.bmap?

Stefan