From: Ric Wheeler Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate Date: Fri, 28 Jan 2011 13:13:16 -0500 Message-ID: <4D43073C.1040100@redhat.com> References: <1289248327-16308-1-git-send-email-josef@redhat.com> <20101109011222.GD2715@dastard> <20101109033038.GF3099@thunk.org> <20101109044242.GH2715@dastard> <20101109214147.GK3099@thunk.org> <20101109234049.GQ2715@dastard> <20110112124431.GP28803@dastard> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Lawrence Greenfield , "Ted Ts'o" , Josef Bacik , linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, xfs@oss.sgi.com, joel.becker@oracle.com, cmm@us.ibm.com, cluster-devel@redhat.com To: Dave Chinner Return-path: In-Reply-To: <20110112124431.GP28803@dastard> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 01/12/2011 07:44 AM, Dave Chinner wrote: > On Tue, Jan 11, 2011 at 04:13:42PM -0500, Lawrence Greenfield wrote: >> On Tue, Nov 9, 2010 at 6:40 PM, Dave Chinner wrote: >>> The historical reason for such behaviour existing in XFS was that in >>> 1997 the CPU and IO latency cost of unwritten extent conversion was >>> significant, > ..... > >>>> (Take for example a trusted cluster filesystem backend that checks the >>>> object checksum before returning any data to the user; and if the >>>> check fails the cluster file system will try to use some other replica >>>> stored on some other server.) >>> IOWs, all they want to do is avoid the unwritten extent conversion >>> overhead. Time has shown that a bad security/performance tradeoff >>> decision was made 13 years ago in XFS, so I see little reason to >>> repeat it for ext4 today.... >> I'd make use of FALLOC_FL_EXPOSE_OLD_DATA. It's not the CPU overhead >> of extent conversion. It's that extent conversion causes more metadata >> operations than what you'd have otherwise, > Yes, that's the "IO latency" part of the cost I mentioned above. > >> which means systems that >> want to use O_DIRECT and make sure the data doesn't go away either >> have to write O_DIRECT|O_DSYNC or need to call fdatasync(). > Seriously, we tell application writers _all the time_ that they > *must* use fsync/fdatasync to guarantee their data is on stable > storage and that they cannot rely on side-effects of filesystem or > storage specific behaviours (like ext3 ordered mode) to do that job > for them. > > You're suggesting that by introducing FALLOC_FL_EXPOSE_OLD_DATA, > applications can rely on filesystem/storage specific behaviour to > guarantee data is on stable storage without the use of > fdatasync/fsync. Wht you describe is definitely storage specific, > because volatile write caches still needs the fdatasync to issue a > cache flush. > > Do you see the same conflict here that I do? > The very concept seems quite "non-enterprise". I also agree that the cost of maintaining extra mount options (and code) for something that no sane end user would ever do seems to be a loss. Why wouldn't you want to convert the punched hole to an unwritten extent? Thanks! Ric