From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753775AbbEHRLX (ORCPT ); Fri, 8 May 2015 13:11:23 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54290 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753529AbbEHRLT (ORCPT ); Fri, 8 May 2015 13:11:19 -0400 Date: Fri, 8 May 2015 10:11:15 -0700 From: Zach Brown To: Sage Weil Cc: Dave Chinner , Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag Message-ID: <20150508171115.GA13050@lenny.home.zabbo.net> References: <1430949612-21356-1-git-send-email-zab@redhat.com> <20150507002617.GJ4327@dastard> <20150507172053.GA659@lenny.home.zabbo.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 07, 2015 at 06:01:23PM -0700, Sage Weil wrote: > On Thu, 7 May 2015, Zach Brown wrote: > > On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote: > > > On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote: > > > > The criteria for using O_NOMTIME is the same as for using O_NOATIME: > > > > owning the file or having the CAP_FOWNER capability. If we're not > > > > comfortable allowing owners to prevent mtime/ctime updates then we > > > > should add a tunable to allow O_NOMTIME. Maybe a mount option? > > > > > > I dislike "turn off safety for performance" options because Joe > > > SpeedRacer will always select performance over safety. > > > > Well, for ceph there's no safety concern. They never use cmtime in > > these files. > > > > So are you suggesting not implementing this and making them rework their > > IO paths to avoid the fs maintaining mtime so that we don't give Joe > > Speedracer more rope? Or are we talking about adding some speed bumps > > that ceph can flip on that might give Joe Speedracer pause? > > I think this is the fundamental question: who do we give the ammunition > to, the user or app writer, or the sysadmin? Yeah, I think this is right. Dave doesn't want the possibility of it bleeding in to installations through irresponsible default use in apps without explicit buy-in from the people responsible for the backups. > [...] > > Or, we can be conservative and require a mount option so that the admin > has to explicitly allow behavior that might break some existing > assumptions about mtime/ctime ('-o user_noatime' I guess?). > > I'm happy either way, so long as in the end an unprivileged ceph daemon > avoids the useless work. In our case we always own the entire mount/disk, > so a mount option is just fine. It seems that the thread has headed towards responding to my suggestion of a possible mount option with an enthusiastic "yes, please, no surprises." So I'll try that. - z