From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by dcvr.yhbt.net (Postfix) with ESMTP id 047571F4B4 for ; Thu, 17 Sep 2020 21:16:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726427AbgIQVQu (ORCPT ); Thu, 17 Sep 2020 17:16:50 -0400 Received: from smtp114.ord1d.emailsrvr.com ([184.106.54.114]:39771 "EHLO smtp114.ord1d.emailsrvr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725900AbgIQVQu (ORCPT ); Thu, 17 Sep 2020 17:16:50 -0400 X-Greylist: delayed 3600 seconds by postgrey-1.27 at vger.kernel.org; Thu, 17 Sep 2020 17:16:49 EDT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=g001.emailsrvr.com; s=20190322-9u7zjiwi; t=1600370490; bh=2HJPC8KV09dvecGouCVtdNyYEBdkXzf+tr7cdC92HLI=; h=Subject:To:From:Date:From; b=ZZJ3t5a4nltlPs1VjpQpsIJp4zQxaj0rW4JpNb8Nac4memExqvDT8IYxgmI7jcQRg wPQJiHj9P3UsBHyUs+JnpxqgsXY8XPmBmjPuJol46qBfH/JCrIS2l5X+qGauW2LlEz KTQcW6qYQBTDuyZfeS1rThb2ednfLfNOi3f7Yw7U= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=xiplink.com; s=20181102-2c3qeqyt; t=1600370490; bh=2HJPC8KV09dvecGouCVtdNyYEBdkXzf+tr7cdC92HLI=; h=Subject:To:From:Date:From; b=wq9+ZphCQJ8SpfweAu49eRl2XrrUbUeprxLvOgoLdoDRb1/zvZinMgTyNiqL77CD4 FueDCZRBNiLG7fuLigMKgpUUmY1KfBDY0jqTovm1DOxSqfM5RKBW/vOIXqCjgt1gHE qIuqyIK6xxYvOGcSfLVR/Psa3KWnl9Zqake3GOzA= X-Auth-ID: mbranchaud@xiplink.com Received: by smtp23.relay.ord1d.emailsrvr.com (Authenticated sender: mbranchaud-AT-xiplink.com) with ESMTPSA id CF21F2039C; Thu, 17 Sep 2020 15:21:29 -0400 (EDT) Subject: Re: [RFC PATCH 2/2] core.fsyncObjectFiles: make the docs less flippant To: =?UTF-8?B?w4Z2YXIgQXJuZmrDtnLDsCBCamFybWFzb24=?= , git@vger.kernel.org Cc: Junio C Hamano , Christoph Hellwig References: <87sgbghdbp.fsf@evledraar.gmail.com> <20200917112830.26606-3-avarab@gmail.com> From: Marc Branchaud Message-ID: <1edd9eb1-365b-b4f7-87f8-3ad35bd7d5be@xiplink.com> Date: Thu, 17 Sep 2020 15:21:29 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200917112830.26606-3-avarab@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Classification-ID: 36143543-7a10-467d-8fa3-aec8e5b40934-1-1 Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On 2020-09-17 7:28 a.m., Ævar Arnfjörð Bjarmason wrote: > As amusing as Linus's original prose[1] is here it doesn't really explain > in any detail to the uninitiated why you would or wouldn't enable > this, and the counter-intuitive reason for why git wouldn't fsync your > precious data. > > So elaborate (a lot) on why this may or may not be needed. This is my > best-effort attempt to summarize the various points raised in the last > ML[2] discussion about this. > > 1. aafe9fbaf4 ("Add config option to enable 'fsync()' of object > files", 2008-06-18) > 2. https://lore.kernel.org/git/20180117184828.31816-1-hch@lst.de/ > > Signed-off-by: Ævar Arnfjörð Bjarmason > --- > Documentation/config/core.txt | 42 ++++++++++++++++++++++++++++++----- > 1 file changed, 36 insertions(+), 6 deletions(-) > > diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt > index 74619a9c03..5b47670c16 100644 > --- a/Documentation/config/core.txt > +++ b/Documentation/config/core.txt > @@ -548,12 +548,42 @@ core.whitespace:: > errors. The default tab width is 8. Allowed values are 1 to 63. > > core.fsyncObjectFiles:: > - This boolean will enable 'fsync()' when writing object files. > -+ > -This is a total waste of time and effort on a filesystem that orders > -data writes properly, but can be useful for filesystems that do not use > -journalling (traditional UNIX filesystems) or that only journal metadata > -and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback"). > + This boolean will enable 'fsync()' when writing loose object > + files. Both the file itself and its containng directory will Typo: containng M. > + be fsynced. > ++ > +When git writes data any required object writes will precede the > +corresponding reference update(s). For example, a > +linkgit:git-receive-pack[1] accepting a push might write a pack or > +loose objects (depending on settings such as `transfer.unpackLimit`). > ++ > +Therefore on a journaled file system which ensures that data is > +flushed to disk in chronological order an fsync shouldn't be > +needed. The loose objects might be lost with a crash, but so will the > +ref update that would have referenced them. Git's own state in such a > +crash will remain consistent. > ++ > +This option exists because that assumption doesn't hold on filesystems > +where the data ordering is not preserved, such as on ext3 and ext4 > +with "data=writeback". On such a filesystem the `rename()` that drops > +the new reference in place might be preserved, but the contents or > +directory entry for the loose object(s) might not have been synced to > +disk. > ++ > +Enabling this option might slow git down by a lot in some > +cases. E.g. in the case of a naïve bulk import tool which might create > +a million loose objects before a final ref update and `gc`. In other > +more common cases such as on a server being pushed to with default > +`transfer.unpackLimit` settings the difference might not be noticable. > ++ > +However, that's highly filesystem-dependent, on some filesystems > +simply calling fsync() might force an unrelated bulk background write > +to be serialized to disk. Such edge cases are the reason this option > +is off by default. That default setting might change in future > +versions. > ++ > +In older versions of git only the descriptor for the file itself was > +fsynced, not its directory entry. > > core.preloadIndex:: > Enable parallel index preload for operations like 'git diff' >