Re: [Qemu-devel] [PATCH v3 2/6] add basic backup support to block driver

[Dietmar Maurer]

> >> This should call bdrv_is_allocated_above like the other block jobs do.
> >> It would be needed later anyway to backup only the topmost image.
> >
> > I do not need that information now, so why do you want that I add dead code?
> 
> I think you do.  You're wasting time reading unallocated clusters and checking
> that they are zero.  bdrv_is_allocated_above gives you the same information
> much more efficiently.  

I thought that just returns information if the data is allocated, or if data
is on backing file?

Or is data guaranteed to be zero if bdrv_is_allocated_above() return 0?

> Do VMA files have to store all the blocks of the source file?

I only use it for full backups currently.

If you want incremental backups, you need to store information about the base 
image,
and this is not the scope of this patch. IMHO incremental backup which 
references to other 
images are a mess. They are difficult to generate and difficult to maintain.

I would prefer to use some kind of Content Addressable Storage, using hashes.
That way you have advantages of full backups and incremental backups 
(deduplication).
Having the additional advantage that we can easily rsync that data to another 
site.