From: Jens Axboe Subject: Re: [PATCH 4/4] ext4: call blkdev_issue_flush on fsync Date: Tue, 20 May 2008 22:14:06 +0200 Message-ID: <20080520201406.GB2512@kernel.dk> References: <482DDA56.6000301@redhat.com> <482DDC04.7020706@redhat.com> <20080520023454.GM15035@mit.edu> <20080520154313.GI16676@shareable.org> <4832F3C6.1050601@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Theodore Tso , ext4 development , linux-kernel Mailing List , linux-fsdevel , Andrew Morton To: Eric Sandeen Return-path: Received: from brick.kernel.dk ([87.55.233.238]:29467 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758436AbYETUOO (ORCPT ); Tue, 20 May 2008 16:14:14 -0400 Content-Disposition: inline In-Reply-To: <4832F3C6.1050601@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, May 20 2008, Eric Sandeen wrote: > Jamie Lokier wrote: > > Theodore Tso wrote: > >> On Fri, May 16, 2008 at 02:09:56PM -0500, Eric Sandeen wrote: > >>> To ensure that bits are truly on-disk after an fsync, > >>> we should call blkdev_issue_flush if barriers are supported. > >> This patch isn't necessary, and in fact will cause a double flush. > >> When you call fsync(), it calls ext4_force_commit(), and we do a the > >> equivalent of a blkdev_issue_flush() today (which is what happenes > >> when you do a submit_bh(WRITE_BARRIER, bh), which is what setting > >> set_ordered_mode(bh) ends up causing. > > > > ISTR fsync() on ext3 did not always force a commit, if in-place data > > writes did not change any metadata. > > I think that might still be true, but I'm still looking through it (in > the background...) > > I tried blktrace to see what was going on but I'm not sure what an "NB" > in the RWBS field means, anyone know? Eric already knows this now, but for the benefit of anyone else that may be curious - it's an empty (data-less) barrier. 'N' is basically 'no data' (eg not a read nor a write) and 'B' is barrier. -- Jens Axboe