[sheepdog] [PATCH v10 06/19] sheep: introduce sparse objects

[Liu Yuan]

On Tue, Jun 03, 2014 at 12:09:00AM +0900, Hitoshi Mitake wrote:
> We don't need to care about a fragmentation when
>  - the object is unlikely to be accessed sequentially, and
>  - the object is read-only.
> 
> In that sense, we can make the objects sparse if they are not data
> objects and writable ones.
> 
> This fixes the problem that sheepdog consumes many disk spaces for
> deleted vdi objects if your filesystem supports FALLOC_FL_PUNCH_HOLE.

I don't understand this patch. If we want sparse objects, we can just stop
preallocation and no need to trim objects in the hottest path. Am I missing
anything in the sense that in one way, we preallocate the whole object, but in
the other we punch hole of objects.

Thanks
Yuan