On Jan 15, 2020, at 1:55 PM, David Howells <dhowells@redhat.com> wrote:
> 
> Andreas Dilger <adilger@dilger.ca> wrote:
> 
>> I think what is needed here is an fadvise/ioctl that tells the filesystem
>> "don't allocate blocks unless actually written" for that file.
> 
> Yeah - and it would probably need to find its way onto disk so that its effect
> is persistent and visible to out-of-kernel tools.
> 
> It would also have to say that blocks of zeros shouldn't be optimised away.

I don't necessarily see that as a requirement, so long as the filesystem
stores a "block" at that offset, but it could dedupe all zero-filled blocks
to the same "zero block".  That still allows saving storage space, while
keeping the semantics of "this block was written into the file" rather than
"there is a hole at this offset".

Cheers, Andreas