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From: Junio C Hamano <gitster@pobox.com>
To: Christoph Hellwig <hch@lst.de>
Cc: "Jeff King" <peff@peff.net>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	git@vger.kernel.org, tytso@mit.edu,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [RFC PATCH 1/2] sha1-file: fsync() loose dir entry when core.fsyncObjectFiles
Date: Thu, 17 Sep 2020 08:37:19 -0700
Message-ID: <xmqqzh5os9cg.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <20200917145653.GA30972@lst.de> (Christoph Hellwig's message of "Thu, 17 Sep 2020 16:56:53 +0200")

Christoph Hellwig <hch@lst.de> 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.

  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-17 18:48 [PATCH] enable core.fsyncObjectFiles by default Christoph Hellwig
2018-01-17 19:04 ` Junio C Hamano
2018-01-17 19:35   ` Christoph Hellwig
2018-01-17 20:05     ` Andreas Schwab
2018-01-17 19:37   ` Matthew Wilcox
2018-01-17 19:42     ` Christoph Hellwig
2018-01-17 21:44   ` Ævar Arnfjörð Bjarmason
2018-01-17 22:07     ` Linus Torvalds
2018-01-17 22:25       ` Linus Torvalds
2018-01-17 23:16       ` Ævar Arnfjörð Bjarmason
2018-01-17 23:42         ` Linus Torvalds
2018-01-17 23:52       ` Theodore Ts'o
2018-01-17 23:57         ` Linus Torvalds
2018-01-18 16:27           ` Christoph Hellwig
2018-01-19 19:08             ` Junio C Hamano
2018-01-20 22:14               ` Theodore Ts'o
2018-01-20 22:27                 ` Junio C Hamano
2018-01-22 15:09                   ` Ævar Arnfjörð Bjarmason
2018-01-22 18:09                     ` Theodore Ts'o
2018-01-23  0:47                       ` Jeff King
2018-01-23  5:45                         ` Theodore Ts'o
2018-01-23 16:17                           ` Jeff King
2018-01-23  0:25                     ` Jeff King
2018-01-21 21:32             ` Chris Mason
2020-09-17 11:06         ` Ævar Arnfjörð Bjarmason
2020-09-17 11:28           ` [RFC PATCH 0/2] should core.fsyncObjectFiles fsync the dir entry + docs Ævar Arnfjörð Bjarmason
2020-09-17 11:28           ` [RFC PATCH 1/2] sha1-file: fsync() loose dir entry when core.fsyncObjectFiles Ævar Arnfjörð Bjarmason
2020-09-17 13:16             ` Jeff King
2020-09-17 15:09               ` Christoph Hellwig
2020-09-17 14:09             ` Christoph Hellwig
2020-09-17 14:55               ` Jeff King
2020-09-17 14:56                 ` Christoph Hellwig
2020-09-17 15:37                   ` Junio C Hamano [this message]
2020-09-17 17:12                     ` Jeff King
2020-09-17 20:37                       ` Taylor Blau
2020-09-22 10:42               ` Ævar Arnfjörð Bjarmason
2020-09-17 20:21             ` Johannes Sixt
2020-09-22  8:24               ` Ævar Arnfjörð Bjarmason
2020-11-19 11:38                 ` Johannes Schindelin
2020-09-17 11:28           ` [RFC PATCH 2/2] core.fsyncObjectFiles: make the docs less flippant Ævar Arnfjörð Bjarmason
2020-09-17 14:12             ` Christoph Hellwig
2020-09-17 15:43             ` Junio C Hamano
2020-09-17 20:15               ` Johannes Sixt
2020-10-08  8:13               ` Johannes Schindelin
2020-10-08 15:57                 ` Ævar Arnfjörð Bjarmason
2020-10-08 18:53                   ` Junio C Hamano
2020-10-09 10:44                   ` Johannes Schindelin
2020-09-17 19:21             ` Marc Branchaud
2020-09-17 14:14           ` [PATCH] enable core.fsyncObjectFiles by default Christoph Hellwig
2020-09-17 15:30           ` Junio C Hamano
2018-01-17 20:55 ` Jeff King
2018-01-17 21:10   ` Christoph Hellwig

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