On Thu, Sep 17, 2020 at 08:37:19AM -0700, Junio C Hamano wrote:

quoted

You need to fsync both to be fully compliant, even if just fsyncing b
will work for most but not all file systems.  The good news is that
for those common file systems the extra fsync of a is almost free.

Back to Ævar's patch, when creating a new loose object, we do these
things:

 1. create temporary file and write the compressed contents to it
    while computing its object name

 2. create the fan-out directory under .git/objects/ if needed

 3. mv temporary file to its final name

and the patch adds open+fsync+close on the fan-out directory.  In
the above exchange with Peff, we learned that open+fsync+close needs
to be done on .git/objects if we created the fan-out directory, too.

Am I reading the above correctly?

That's my understanding. It gets trickier with refs (which I think we
also ought to consider fsyncing), as we may create arbitrarily deep
hierarchies (so we'd have to keep track of which parts got created, or
just conservatively fsync up the whole hierarchy).

It gets a lot easier if we move to reftables that have a more
predictable file/disk structure.

-Peff