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From: Jeff King <peff@peff.net>
To: Christoph Hellwig <hch@lst.de>
Cc: git@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] enable core.fsyncObjectFiles by default
Date: Wed, 17 Jan 2018 15:55:09 -0500	[thread overview]
Message-ID: <20180117205509.GA14828@sigill.intra.peff.net> (raw)
In-Reply-To: <20180117184828.31816-1-hch@lst.de>

On Wed, Jan 17, 2018 at 07:48:28PM +0100, Christoph Hellwig wrote:

> fsync is required for data integrity as there is no gurantee that
> data makes it to disk at any specified time without it.  Even for
> ext3 with data=ordered mode the file system will only commit all
> data at some point in time that is not guaranteed.
> 
> I've lost data on development machines with various times countless
> times due to the lack of this option, and now lost trees on a
> git server with ext4 as well yesterday.  It's time to make git
> safe by default.

I'm definitely sympathetic, and I've contemplated a patch like this a
few times. But I'm not sure we're "safe by default" here after this
patch. In particular:

  1. This covers only loose objects. We generally sync pack writes
     already, so we're covered there. But we do not sync ref updates at
     all, which we'd probably want to in a default-safe setup (a common
     post-crash symptom I've seen is zero-length ref files).

  2. Is it sufficient to fsync() the individual file's descriptors?
     We often do other filesystem operations (like hardlinking or
     renaming) that also need to be committed to disk before an
     operation can be considered saved.

  3. Related to (2), we often care about the order of metadata commits.
     E.g., a common sequence is:

       a. Write object contents to tempfile.

       b. rename() or hardlink tempfile to final name.

       c. Write object name into ref.lock file.

       d. rename() ref.lock to ref

     If we see (d) but not (b), then the result is a corrupted
     repository. Is this guaranteed by ext4 journaling with
     data=ordered?

It may be that data=ordered gets us what we need for (2) and (3). But I
think at the very least we should consider fsyncing ref updates based on
a config option, too.

-Peff

  parent reply	other threads:[~2018-01-17 20:55 UTC|newest]

Thread overview: 60+ 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
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 [this message]
2018-01-17 21:10   ` Christoph Hellwig
  -- strict thread matches above, loose matches on Subject: below --
2015-06-23 21:57 [PATCH] Enable " Stefan Beller
2015-06-23 22:21 ` Junio C Hamano
2015-06-23 23:29   ` Theodore Ts'o
2015-06-24  5:32     ` Junio C Hamano
2015-06-24 14:30       ` Theodore Ts'o
2015-06-24  1:07 ` Duy Nguyen
2015-06-24  3:37 ` Jeff King
2015-06-24  5:20   ` Junio C Hamano

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