On Fri, Oct 12, 2018 at 2:21 AM Christoph Hellwig <hch@xxxxxx> wrote: > > On Thu, Oct 11, 2018 at 01:38:34PM -0500, Eric Sandeen wrote: > > > The different behavior between filesystems was confusing customers so > > > we had to align them, then the question was which default to pick. > > > Honestly, we came to the decision to bring ext4 in line with the xfs > > > behavior because we thought that would be easier than the alternative. > > > Dave and Christoph made repeated arguments that DAX is just a hidden > > > performance optimization that no application should rely on, so we > > > went the path of least resistance and changed the ext4 default. > > > > Ok, well, I guess we'd better reconcile "it's a hidden performance hint" > > with "if the administrator asked they must receive..." before making this > > change... cc: hch for bonus input. > > I don't really care too mouch on the mount options, the important bit > was the application behavior. > > I fully agree with Dan that we should have the same behavior for every > file system, though. One factor that might influence this is how we expect users to detect whether or not DAX is being used, and whether that can vary per-inode within a filesystem. If we choose to only have the mount option then I agree that a hard failure when -o dax doesn't work seems fine. And of course keeping the filesystems behaving the same is desirable. If we eventually do go back to having a per-inode DAX option, though, the mount option becomes a hint as to what the default behavior is, and the user will need another way to detect whether or not DAX is being used for a given inode. In that case having the mount option fail loudly isn't as important because all we've really changed is the filesystem's default, and the application will still need a consistent way of detecting whether the inode they are actually using is DAX or not. I'm not sure if per-inode DAX is still a goal for anyone. If not, then sure, using the DAX mount option as the one source of truth and making it a hard failure when it doesn't work seems reasonable.
