From owner-freebsd-fs@FreeBSD.ORG Tue Aug 31 23:07:31 2010 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 16FE110656A3 for ; Tue, 31 Aug 2010 23:07:31 +0000 (UTC) (envelope-from cracauer@koef.zs64.net) Received: from koef.zs64.net (koef.zs64.net [IPv6:2001:470:1f0b:105e::1e6]) by mx1.freebsd.org (Postfix) with ESMTP id CC23A8FC15 for ; Tue, 31 Aug 2010 23:07:30 +0000 (UTC) Received: from koef.zs64.net (koef.zs64.net [IPv6:2001:470:1f0b:105e::1e6]) by koef.zs64.net (8.14.4/8.14.4) with ESMTP id o7VN7Tkg040888; Wed, 1 Sep 2010 01:07:29 +0200 (CEST) (envelope-from cracauer@koef.zs64.net) Received: (from cracauer@localhost) by koef.zs64.net (8.14.4/8.14.4/Submit) id o7VN7S0W040565; Tue, 31 Aug 2010 19:07:28 -0400 (EDT) (envelope-from cracauer) Date: Tue, 31 Aug 2010 19:07:28 -0400 From: Martin Cracauer To: Benjamin Kaduk Message-ID: <20100831230728.GA36384@cons.org> References: <20100830225841.GA9363@cons.org> <20100831160840.GA74125@icarus.home.lan> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Cc: freebsd-fs@freebsd.org, Martin Cracauer Subject: Re: fsync(2) and on-disk write-back cache X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 31 Aug 2010 23:07:31 -0000 Benjamin Kaduk wrote on Tue, Aug 31, 2010 at 12:12:04PM -0400: > On Tue, 31 Aug 2010, Jeremy Chadwick wrote: > > >On Mon, Aug 30, 2010 at 06:58:42PM -0400, Martin Cracauer wrote: > >>I always assumed the answer to this question is "of course": > >> > >>When doing an fsync (waiting for the commit), do we actually tell the > >>disk to flush the on-disk write-back cache (if that is in use) to the > >>platters? > >> > >>I just went down some code paths in both FreeBSD and Linux and in both > >>cases the paths for fsync quickly disappear in the generic > >>block-by-block flushing code that is also used for regular (non-fsync) > >>flushing. I didn't see anything aware of the on-disk cache. > > > >I don't have an authoritative answer to your question, but this thread > >seems to imply there's a relation between fsync() and an intentional > >disk flush (BIO_FLUSH). I'm sure when BIO_FLUSH is called depends on > >the filesystem as well. I just went the for-dummies way and annotated all relevant BIO_FLUSH places with debug print statements. They don't seem to be called when doing an fsync on a file in a local filesystem. ufs (no softupdates) -> old-style SCSI disk. I'll snoop around some more, try it on ZFS/SATA and do some timing tests. > It is probably also worth noting that disks have been known to lie about > having actually flushed bits from their internal cache to the platter. I know, that's why the whole question is a little academic. Martin -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Martin Cracauer http://www.cons.org/cracauer/