Re: [PATCH] ext4: introduce per-inode DAX flag

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 24, 2017 at 02:20:57PM -0400, Theodore Ts'o wrote:
> The counter-argument is that system administrators do need to have a
> way to signal that they would like the file system to "do something
> different" on a per-file basis, and no one else has come up with
> another way of doing things.  Furthermore, it would be highly
> desirable if the system adminisator can provide this per-file system
> hint with requiring changes to the application.  (For example, by
> adding madvise/fadvise hints.)

We can always add some sort of inode or subtree advice.  It's just
the binary flag that encodes a specific implementation that is
very bad in the long run.

> Is that a fair summary of the argument?

Otherwise yet.

> I have two additional questions I'd like to ask at this point.
> 
> 1)  Has there been any other difficulty that XFS has had due to the
> fact that they have this DAX flag added?  e.g., are there any
> operational, or practical code maintainability issues at stake here?
> Or is this mostly an design philosophy debate?

It hasn't yet.  It will create really annoying problems once we
use raw DAX access for metadata, which I had prototype a while ago
and plan to finnally get in in the next months.

> 2) Are there any users using the DAX flag with XFS such that, if XFS
> were to remove the DAX flag support, those users would complain
> bitterly?

I don't know of anyone that actually uses the flag.  If someone did
that would probably run into problems like changing that changing it
on a file that's currently mmaped would crash an burn badly.