On Wed, Jul 26, 2017 at 10:20 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Wed, Jul 26, 2017 at 10:11:08AM -0700, Dan Williams wrote: >> Until HMAT came along we had no data in the kernel how to pick a sane >> default, but we could now very easily make a "if pmem performance < >> dram, disable dax by default" policy in the kernel. > > I'd rather do it the other way around - if HMAT is present and > pmem performance >= dram use dax. Else require the explicit -o dax > for now to enable it. If an explicit -o nodax is specified disable > DAX even if HMAT says it is faster. Silently turn on DAX if HMAT says its ok? I think we would instead want a "-o autodax" for that case and then "-o dax" and "-o nodax" for the force cases. >> The question for this patch is do we want to add yet another >> filesystem that adds "-o dax" or require use of per-inode flags to >> enable dax. > > Please stick to the mount option. After spending a lot of time with > DAX and various memory techologies I'm pretty confident that the inode > flag is the wrong thing to do. I think it's easier to administer than the dax mount option. If someone wants dax on only in a sub-tree they can set the flag on that parent directory and have a policy in dax filesystems that children inherit the dax policy from the parent. That seems a better administrative model than trying to get it all right globally at mount time.
