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From: Theodore Ts'o <tytso@mit.edu>
To: Junio C Hamano <gitster@pobox.com>
Cc: Stefan Beller <sbeller@google.com>,
	git@vger.kernel.org, peff@peff.net,
	torvalds@linux-foundation.org
Subject: Re: [PATCH] Enable core.fsyncObjectFiles by default
Date: Wed, 24 Jun 2015 10:30:07 -0400
Message-ID: <20150624143007.GC14324@thunk.org> (raw)
In-Reply-To: <xmqqegl1brjb.fsf@gitster.dls.corp.google.com>

On Tue, Jun 23, 2015 at 10:32:08PM -0700, Junio C Hamano wrote:
> 
> Regarding loose object files, given that we write to a temporary,
> optionally fsync, close and then move to the final name, would we
> still see partially written file if we omit the fsync, or would the
> corruption be limited to either empty or missing?

*Most* of the time the corruption will be an empty or missing file.
It's possible that the file could be partially written.  This is a
relatively low-probability event, with the probability going up if the
object file is large, and/or if the system is under memory pressure.

> The reason I am wondering is because the codepath to create an
> object (i.e. "update-index --add", "hash-object -w", or "add") first
> checks if a packed or a loose object file _exists_ and if so
> bypasses writing the same thing anew, but the existence check for a
> loose object is to merely making sure that access(F_OK) (and
> optionally utime()) succeeds.  If the potential breakage is limited
> to truncation to empty, then we could replace it with stat(2) and
> st.st_size check, as no loose object file can be empty.

It would certainly be a good thing to do a st_size check; it can't
possible hurt, and it will catch a large number of failures after a
power failure.  I could also imagine some hueristics that force an
fsync if the object file is larger than a certain size (say, 4k if you
are very paranoid, a few hundred kilobytes if you are less so), but
past a certain point, it might be better just to tell the user to use
fsyncObjectFiles and be done with it.

						- Ted

  reply index

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-06-23 21:57 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 [this message]
2015-06-24  1:07 ` Duy Nguyen
2015-06-24  3:37 ` Jeff King
2015-06-24  5:20   ` Junio C Hamano
2018-01-17 18:48 [PATCH] enable " 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 14:14           ` 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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