Re: [PATCH v8 1/1] f2fs: dax: implement direct access

[Dan Williams]

On Wed, Jul 26, 2017 at 10:01 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Wed, Jul 26, 2017 at 09:53:07AM -0700, Dan Williams wrote:
>> It allows for opt-in for applications, or administrators of those
>> applications, that know the type of access.
>
> That's BS.  We need to provide the best possible way to access the
> media to an application.  And whether that's DAX or the page cache
> is an implementation detail that should not matter to the application.
>
> Which doesn't mean there shouldn't be ways to override the default
> that the kernel chose based on hardware details, but it's certainly
> not something for the application to hardcode, but something for
> the adminstrator to decide.

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.

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.