From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI,T_RP_MATCHES_RCVD shortcircuit=no autolearn=ham autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 2B21F1F406 for ; Wed, 17 Jan 2018 21:10:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753626AbeAQVKP (ORCPT ); Wed, 17 Jan 2018 16:10:15 -0500 Received: from verein.lst.de ([213.95.11.211]:58037 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753462AbeAQVKN (ORCPT ); Wed, 17 Jan 2018 16:10:13 -0500 Received: by newverein.lst.de (Postfix, from userid 2407) id DBE8968DBC; Wed, 17 Jan 2018 22:10:11 +0100 (CET) Date: Wed, 17 Jan 2018 22:10:11 +0100 From: Christoph Hellwig To: Jeff King Cc: Christoph Hellwig , git@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] enable core.fsyncObjectFiles by default Message-ID: <20180117211011.GA355@lst.de> References: <20180117184828.31816-1-hch@lst.de> <20180117205509.GA14828@sigill.intra.peff.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180117205509.GA14828@sigill.intra.peff.net> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Wed, Jan 17, 2018 at 03:55:09PM -0500, Jeff King wrote: > 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). I've not seen them myself yet, but yes, they need an fsync. > 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. No, for metadata operations we need to fsync the directory as well. > 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 is not generally guranteed by Linux file system semantics. Various file system will actually start writeback of file data before rename, but not actually wait on it.