From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Theodore Ts'o <tytso@mit.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Junio C Hamano <gitster@pobox.com>,
Christoph Hellwig <hch@lst.de>,
Git Mailing List <git@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Jacob Vosmaer <jacob@gitlab.com>
Subject: Re: [PATCH] enable core.fsyncObjectFiles by default
Date: Thu, 17 Sep 2020 13:06:50 +0200 [thread overview]
Message-ID: <87sgbghdbp.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <20180117235220.GD6948@thunk.org>
On Thu, Jan 18 2018, Theodore Ts'o wrote:
> On Wed, Jan 17, 2018 at 02:07:22PM -0800, Linus Torvalds wrote:
>>
>> Now re-do the test while another process writes to a totally unrelated
>> a huge file (say, do a ISO file copy or something).
>>
>> That was the thing that several filesystems get completely and
>> horribly wrong. Generally _particularly_ the logging filesystems that
>> don't even need the fsync, because they use a single log for
>> everything (so fsync serializes all the writes, not just the writes to
>> the one file it's fsync'ing).
>
> Well, let's be fair; this is something *ext3* got wrong, and it was
> the default file system back them. All of the modern file systems now
> do delayed allocation, which means that an fsync of one file doesn't
> actually imply an fsync of another file. Hence...
>
>> The original git design was very much to write each object file
>> without any syncing, because they don't matter since a new object file
>> - by definition - isn't really reachable. Then sync before writing the
>> index file or a new ref.
>
> This isn't really safe any more. Yes, there's a single log. But
> files which are subject to delayed allocation are in the page cache,
> and just because you fsync the index file doesn't mean that the object
> file is now written to disk. It was true for ext3, but it's not true
> for ext4, xfs, btrfs, etc.
>
> The good news is that if you have another process downloading a huge
> ISO image, the fsync of the index file won't force the ISO file to be
> written out. The bad news is that it won't force out the other git
> object files, either.
>
> Now, there is a potential downside of fsync'ing each object file, and
> that is the cost of doing a CACHE FLUSH on a HDD is non-trivial, and
> even on a SSD, it's not optimal to call CACHE FLUSH thousands of times
> in a second. So if you are creating thousands of tiny files, and you
> fsync each one, each fsync(2) call is a serializing instruction, which
> means it won't return until that one file is written to disk. If you
> are writing lots of small files, and you are using a HDD, you'll be
> bottlenecked to around 30 files per second on a 5400 RPM HDD, and this
> is true regardless of what file system you use, because the bottle
> neck is the CACHE FLUSH operation, and how you organize the metadata
> and how you do the block allocation, is largely lost in the noise
> compared to the CACHE FLUSH command, which serializes everything.
>
> There are solutions to this; you could simply not call fsync(2) a
> thousand times, and instead write a pack file, and call fsync once on
> the pack file. That's probably the smartest approach.
>
> You could also create a thousand threads, and call fsync(2) on those
> thousand threads at roughly the same time. Or you could use a
> bleeding edge kernel with the latest AIO patch, and use the newly
> added IOCB_CMD_FSYNC support.
>
> But I'd simply recommend writing a pack and fsync'ing the pack,
> instead of trying to write a gazillion object files. (git-repack -A,
> I'm looking at you....)
>
> - Ted
[I didn't find an ideal message to reply to in this thread, but this
seemed to probably be the best]
Just an update on this since I went back and looked at this thread,
GitLab about ~1yr ago turned on core.fsyncObjectFiles=true by
default.
The reason is detailed in [1], tl;dr: empty loose object file issue on
ext4 allegedly caused by a lack of core.fsyncObjectFiles=true, but I
didn't do any root cause analysis. Just noting it here for for future
reference.
1. https://gitlab.com/gitlab-org/gitlab-foss/-/issues/51680#note_180508774
next prev parent reply other threads:[~2020-09-17 11:07 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 [this message]
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
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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