On Wed, Mar 10, 2021 at 08:36:06AM -0500, Neal Gompa wrote: > On Wed, Mar 10, 2021 at 8:02 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Wed, Mar 10, 2021 at 07:30:41AM -0500, Neal Gompa wrote: > > > Forgive my ignorance, but is there a reason why this isn't wired up to > > > Btrfs at the same time? It seems weird to me that adding a feature > > > > btrfs doesn't support DAX. only ext2, ext4, XFS and FUSE have DAX support. > > > > If you think about it, btrfs and DAX are diametrically opposite things. > > DAX is about giving raw access to the hardware. btrfs is about offering > > extra value (RAID, checksums, ...), none of which can be done if the > > filesystem isn't in the read/write path. > > > > That's why there's no DAX support in btrfs. If you want DAX, you have > > to give up all the features you like in btrfs. So you may as well use > > a different filesystem. > > So does that mean that DAX is incompatible with those filesystems when > layered on DM (e.g. through LVM)? Yes. It might be possible to work through RAID-0 or read-only through RAID-1, but I'm not sure anybody's bothered to do that work. > Also, based on what you're saying, that means that DAX'd resources > would not be able to use reflinks on XFS, right? That'd put it in > similar territory as swap files on Btrfs, I would think. You can use DAX with reflinks because the CPU can do read-only mmaps. On a write fault, we break the reflink, copy the data and put in a writable PTE.
