Re: [Bug-tar] Detection of sparse files is broken on btrfs

[Paul Eggert]

On 01/08/2018 08:54 AM, Joerg Schilling wrote:
> The most important fact however is that allocating spade happens before you
> copy data into that space.

Certainly users need the ability to make sure there's enough room before 
starting to copy, and POSIX allows for that with posix_fallocate. 
However, I don't see any POSIX requirement that st_blocks has anything 
to do with that room. For example, POSIX doesn't specify the size of the 
units that st_blocks uses, and it even allows the unit size to vary 
depending on the file. One cannot really deduce much of anything from 
st_blocks, if the goal is portability to any POSIX implementation.

> I cannot see any reason why deduplication between different
> filesystems that share a common dataset should be alllowed to be visible at
> user level.
I can. Filesystem implementers are trying to shoehorn their modern 
designs into the POSIX st_blocks interface which is loosely-specified 
and was inspired by an antiquated design. At some point, it is 
reasonable for them to give up with the pretense and just put 0 in 
st_blocks, since whatever number is put in there is in some sense 
"wrong" anyway.