Re: [RFC PATCH 1/2] sha1-file: fsync() loose dir entry when core.fsyncObjectFiles

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 17, 2020, 15:37 UTC
Message-ID: <xmqqzh5os9cg.fsf@gitster.c.googlers.com>
In-Reply-To: <20200917145653.GA30972@lst.de>

Christoph Hellwig <hch@lst.de> writes:

Show 26 quoted lines

> 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.