Re: [Qemu-discuss] What does "COW" mean?

[Kevin Wolf]

Am 03.09.2014 um 04:06 hat shhuiw geschrieben:
> 
> 
> At 2014-09-02 04:33:50, "shhuiw" <address@hidden> wrote:
> >
> >
> >Hi,
> >
> >I'm new to qemu community, and I'm trying the COW image format (old but 
> >simple:-).
> >I have read through its source code, and didn't find anything about 'copy on 
> >write'.
> >I wonder wthat "COW" stands for?
> 
> Sorry for my unclear expression.
> I mean when copy-on-write happens if COW image format is used, and how the 
> COW driver code handles cop-on-write?

It is what happens in cow_write(). Initially the sector is marked as
unallocated in the bitmap and you don't need space for it. Any reads are
redirected to the backing file. As soon as you write to it, it gets
marked as allocated and written into the image file. This is the COW
step.

Kevin