From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH] ext4: introduce per-inode DAX flag Date: Fri, 25 Aug 2017 00:54:15 -0700 Message-ID: <20170825075415.GA748@infradead.org> References: <1501690186-17607-1-git-send-email-lczerner@redhat.com> <20170805084601.GA12425@infradead.org> <20170807121212.nuiho4afmdjf5vsd@rh_laptop> <20170808090016.lxfbcrhoybmyfvlc@rh_laptop> <20170811100147.GB7064@infradead.org> <20170811121132.bj5y77scrvkgy7uo@rh_laptop> <20170811125849.GA15300@infradead.org> <20170811134130.o46y5jpekrpj5qvw@rh_laptop> <20170824182057.amdirlrbugezrahy@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Christoph Hellwig , linux-ext4@vger.kernel.org, Lukas Czerner , linux-xfs@vger.kernel.org To: Theodore Ts'o Return-path: Received: from bombadil.infradead.org ([65.50.211.133]:58280 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754521AbdHYHyQ (ORCPT ); Fri, 25 Aug 2017 03:54:16 -0400 Content-Disposition: inline In-Reply-To: <20170824182057.amdirlrbugezrahy@thunk.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, Aug 24, 2017 at 02:20:57PM -0400, Theodore Ts'o wrote: > The counter-argument is that system administrators do need to have a > way to signal that they would like the file system to "do something > different" on a per-file basis, and no one else has come up with > another way of doing things. Furthermore, it would be highly > desirable if the system adminisator can provide this per-file system > hint with requiring changes to the application. (For example, by > adding madvise/fadvise hints.) We can always add some sort of inode or subtree advice. It's just the binary flag that encodes a specific implementation that is very bad in the long run. > Is that a fair summary of the argument? Otherwise yet. > I have two additional questions I'd like to ask at this point. > > 1) Has there been any other difficulty that XFS has had due to the > fact that they have this DAX flag added? e.g., are there any > operational, or practical code maintainability issues at stake here? > Or is this mostly an design philosophy debate? It hasn't yet. It will create really annoying problems once we use raw DAX access for metadata, which I had prototype a while ago and plan to finnally get in in the next months. > 2) Are there any users using the DAX flag with XFS such that, if XFS > were to remove the DAX flag support, those users would complain > bitterly? I don't know of anyone that actually uses the flag. If someone did that would probably run into problems like changing that changing it on a file that's currently mmaped would crash an burn badly.