Christoph Hellwig <hch@xxxxxx> writes:
> On Thu, Sep 17, 2020 at 10:55:23AM -0400, Jeff King wrote:
>> On Thu, Sep 17, 2020 at 04:09:12PM +0200, Christoph Hellwig wrote:
>>
>> > On Thu, Sep 17, 2020 at 01:28:29PM +0200, Ævar Arnfjörð Bjarmason wrote:
>> > > Change the behavior of core.fsyncObjectFiles to also sync the
>> > > directory entry. I don't have a case where this broke, just going by
>> > > paranoia and the fsync(2) manual page's guarantees about its behavior.
>> >
>> > It is not just paranoia, but indeed what is required from the standards
>> > POV. At least for many Linux file systems your second fsync will be
>> > very cheap (basically a NULL syscall) as the log has alredy been forced
>> > all the way by the first one, but you can't rely on that.
>>
>> Is it sufficient to fsync() just the surrounding directory? I.e., if I
>> do:
>>
>> mkdir("a");
>> mkdir("a/b");
>> open("a/b/c", O_WRONLY);
>>
>> is it enough to fsync() a descriptor pointing to "a/b", or should I
>> also do "a"?
>
> 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?
Thanks.
