On Mon, Feb 8, 2016 at 12:18 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: [..] >> Setting aside the current block zeroing problem you seem to assuming >> that DAX will always be faster and that may not be true at a media >> level. Waiting years for some applications to determine if DAX makes >> sense for their use case seems completely reasonable. In the meantime >> the apps that are already making these changes want to know that a DAX >> mapping request has not silently dropped backed to page cache. They >> also want to know if they successfully jumped through all the hoops to >> get a larger than pte mapping. >> >> I agree it is useful to be able to force DAX on an unmodified >> application to see what happens, and it follows that if those >> applications want to run in that mode they will need functional >> fsync()... >> >> I would feel better if we were talking about specific applications and >> performance numbers to know if forcing DAX on application is a debug >> facility or a production level capability. You seem to have already >> made that determination and I'm curious what I'm missing. > > I'm not setting any policy here at all. This whole argument is > based around the DAX mount option doing "global fs enable or > silently turning it off" and the application not knowing about that. > > The whole point of having a persistent per-inode DAX flags is that > it is a policy mechanism, not a policy. The application can, if it > is DAX aware, directly control whether DAX is used on a file or not. > The application can even query and clear that persistent inode flag > if it is configured not to (or cannot) use DAX. > > If the filesystem cannot support DAX, then we can error out attempts > to set the DAX flag and then the app knows DAX is not available. > i.e. the attempt to set policy failed. If the flag is set, then the > inode will *always* use DAX - there is no "fall back to page cache" > when DAX is enabled. > > If the applicaiton is not DAX aware, then the admin can control the > DAX policy by manipulating these flags themselves, and hence control > whether DAX is used by the application or not. > > If you think I'm dictating policy for DAX users and application, > then you haven't understood anything I've previously said about why > the DAX mount option needs to die before any of this is considered > production ready. DAX is not an opaque "all or nothing" option. XFS > will provide apps and admins with fine-grained, persistent, > discoverable policy flags to allow admins and applications to set > DAX policies however they see fit. This simply cannot be done if the > only knob you have is a mount option that may or may not stick. I agree the mount option needs to die, and I fully grok the reasoning. What I'm concerned with is that a system using fully-DAX-aware applications is forced to incur the overhead of maintaining *sync semantics, periodic sync(2) in particular, even if it is not relying on those semantics. However, like I said in my other mail, we can solve that with alternate interfaces to persistent memory if that becomes an issue and not require that "disable *sync" capability to come through DAX. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
