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

[Stefan Hajnoczi]

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

The block/cow.c driver is inefficient because it does I/O for each
bitmap set/test operation.  I think doing this more efficiently means
basically rewriting the bitmap code to keep a writethrough bitmap in
memory.

Regarding the file header, the msize is not really useful - there is
no interface to read it and no feature makes use of msize.  The
sector_size field could also be dropped.  The true sector size does
from the underlying storage that contains this image file.  Especially
in the cache=none (O_DIRECT) case we need to honor the underlying
sector size and I'm not sure the sector_size field helps.

Stefan