Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752357AbbEHCph (ORCPT ); Thu, 7 May 2015 22:45:37 -0400 Received: from ipmail06.adl2.internode.on.net ([150.101.137.129]:12596 "EHLO ipmail06.adl2.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751652AbbEHCpb (ORCPT ); Thu, 7 May 2015 22:45:31 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: A2BECADdIUxVPPDOLHlcgw6BMoJQg3ytKgEBBplOAgIBAQKBQU0BAQEBAQEHAQEBAUABP4QhAQEEOhwjEAgDDgoJJQ8FDRgDBxoTiBcDEcF9DYUvAQEIAgEfGIV/hSOCTYFMbAeELQWbOYFUjF6CfoZsgQSDJywxgQMHgTwBAQE Date: Fri, 8 May 2015 12:42:59 +1000 From: Dave Chinner To: Andy Lutomirski Cc: Richard Weinberger , Zach Brown , Alexander Viro , Sage Weil , linux-fsdevel , LKML , "open list:ABI/API" Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag Message-ID: <20150508024259.GL4327@dastard> 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/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2445 Lines: 54 On Thu, May 07, 2015 at 12:53:46PM -0700, Andy Lutomirski wrote: > On Thu, May 7, 2015 at 12:09 PM, Richard Weinberger > wrote: > > On Thu, May 7, 2015 at 7:20 PM, 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: > >>> > Add the O_NOMTIME flag which prevents mtime from being updated which can > >>> > greatly reduce the IO overhead of writes to allocated and initialized > >>> > regions of files. > >>> > >>> Hmmm. How do backup programs now work out if the file has changed > >>> and hence needs copying again? ie. applications using this will > >>> break other critical infrastructure in subtle ways. > >> > >> By using backup infrastructure that doesn't use cmtime. Like btrfs > >> send/recv. Or application level backups that know how to do > >> incrementals from metadata in giant database files, say, without > >> walking, comparing, and copying the entire thing. > > > > But how can Joey random user know that some of his > > applications are using O_NOMTIME and his KISS backup > > program does no longer function as expected? > > > > Joey random user can't have a working KISS backup anyway, though, > because we screw up mtime updates on mmap writes. I have patches > gathering dust that fix that, though. They are close enough to be good for backup purposes. The mtime only need change once per backup period - it doesn't need to be millisecond accurate. Yes, I know you needed that changed for different reasons (avoid variable page fault latency), but it doesn't matter for once-a-day or even once-an-hour incremental backup scans. Besides, anyone who cares about accurate backups is doing a backup from a snapshot so they data and metadata is consistent across the entire backup. And that makes worries about mmap and mtime completely irrelevant because a snapshot freezes the filesystem and hence cleans all the mapped pages. Once the snapshot is taken the next mmap write will trigger a page fault and so change the mtime and it will be picked up in the next backup scan... Cheers, Dave. -- Dave Chinner david@fromorbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/