[libvirt] [PATCH 3/3] Add volume zeroing

[Dave Allan]

On 02/15/2010 04:47 PM, Paolo Bonzini wrote:
> On 02/15/2010 10:29 PM, David Allan wrote:
>> * If the volume is a sparse file and the fiemap ioctl is available,
>> use fiemap to locate the volume's extents.
>
> What about for a sparse file doing just
>
> ftruncate (fd, 0);
> ftruncate (fd, st.st_size);
>
> ?
>
> It's already sparse, it doesn't hurt to make it _more_ sparse.
>
> Paolo

That's a good point.  The only thing that makes me hesitate is that I'm 
not certain that the file is guaranteed to contain zeros following the 
second call to ftruncate, and I'm having trouble coming up with the 
relevant spec at the moment.

This question might be academic in any case, since the volume zeroing is 
part of volume deletion, so we're talking about the case of a volume 
represented by a regular file on a backend that doesn't support volume 
delete.  I'm not sure such a case exists.  If that's so, we could just 
skip the whole zeroing operation entirely for regular files.

Dave