On 13:02 10/03, Matthew Wilcox 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. DAX on btrfs has been attempted[1]. Of course, we could not have checksums or multi-device with it. However, got stuck on associating a shared extent on the same page mapping: basically the TODO above dax_associate_entry(). Shiyang has proposed a way to disassociate existing mapping, but I don't think that is the best solution. DAX for CoW will not work until we have a way of mapping a page to multiple inodes (page->mapping), which will convert a 1-N inode-page mapping to M-N inode-page mapping. [1] https://lore.kernel.org/linux-btrfs/20190429172649.8288-1-rgoldwyn@xxxxxxx/ -- Goldwyn
