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

[Dan Williams]

On Wed, Jul 26, 2017 at 1:11 PM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Wed, Jul 26, 2017 at 12:16:11PM -0700, Dan Williams wrote:
>> Silently turn on DAX if HMAT says its ok?
>
> Yes, absolutely.  I want my system to do the right thing by default,
> and if HMAT says bypassing the page cache is a clear advatange it
> should be the default.
>
>> I think we would instead
>> want a "-o autodax" for that case and then "-o dax" and "-o nodax" for
>> the force cases.
>
> Why?
>

I'm worried about the case where HMAT says pmem >= dram performance,
but dax semantics like disabling delayed allocation and
dirty-cacheline tracking end up hurting performance, but I guess we
can handle that on a case by case basis with targeted kernel
optimizations.

>> 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.
>
> And why exactly? If DAX is faster for file a in directory X it will
> be just as fast for a file b in directory Y.

So I want the inode setting for the pmem < dram performance case where
I know that access patterns of the application using file b in
directory Y can still yield better performance without page cache. For
example, the working set is larger than dram capacity.