Git Mailing List Archive on lore.kernel.org
 help / color / Atom feed
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
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

  parent reply index

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-17 18:48 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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87sgbghdbp.fsf@evledraar.gmail.com \
    --to=avarab@gmail.com \
    --cc=git@vger.kernel.org \
    --cc=gitster@pobox.com \
    --cc=hch@lst.de \
    --cc=jacob@gitlab.com \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=torvalds@linux-foundation.org \
    --cc=tytso@mit.edu \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Git Mailing List Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/git/0 git/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 git git/ https://lore.kernel.org/git \
		git@vger.kernel.org
	public-inbox-index git

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.git


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git