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

[Paul Eggert]

On 01/08/2018 10:50 AM, Joerg Schilling wrote
> So the real problem is not what exact value you may expect but rather whether
> btrfs behaves inconsistent because you get different results after calling
> fsync().

POSIX does not require that st_nblocks remain constant across any system 
call. It doesn't even require that it remain constant if you merely call 
stat twice on the same file, without doing anything else in between. So 
I agree with you that it's irrelevant whether fsync or sync is called in 
the interim. Where we disagree is that I don't think st_nblocks must 
remain constant when a file is not modified. No such requirement is in 
POSIX.