[LSF/MM TOPIC] Standardizing semantics around the per-file DAX flag

[LSF/MM TOPIC] Standardizing semantics around the per-file DAX flag

[Theodore Y. Ts'o]

There's been a long-term disagreement about how the per-file DAX flag
should work.

       * Should it exist at all?
       * What happens when the DAX flag is cleared?
           * Should it be not allowed and return an error?
	       (Or maybe only if the file is otherwise opened anywhere in the system?)
	   * Should it only takes effect when the file system is unmounted,
	     or when the inode drops out of the inode cache?
	* Should we remove the flag entirely and make it be something the
	  system automagically infers?

I had hoped consensus would be achieved before the ext4 per-file DAX
flag lands, but it hasn't for a *long* time.  Technically the DAX flag
is "experimental", which technically means it could be removed ---
although I suspect at this point, it would break some userspace, so
our options about how to adjust the semantics of the flag are probably
constrained.

						- Ted
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Re: [LSF/MM TOPIC] Standardizing semantics around the per-file DAX flag

[Jan Kara]

On Wed 13-02-19 11:40:39, Theodore Y. Ts'o wrote:
> There's been a long-term disagreement about how the per-file DAX flag
> should work.
> 
>        * Should it exist at all?
>        * What happens when the DAX flag is cleared?
>            * Should it be not allowed and return an error?
> 	       (Or maybe only if the file is otherwise opened anywhere in the system?)
> 	   * Should it only takes effect when the file system is unmounted,
> 	     or when the inode drops out of the inode cache?
> 	* Should we remove the flag entirely and make it be something the
> 	  system automagically infers?
> 
> I had hoped consensus would be achieved before the ext4 per-file DAX
> flag lands, but it hasn't for a *long* time.  Technically the DAX flag
> is "experimental", which technically means it could be removed ---
> although I suspect at this point, it would break some userspace, so
> our options about how to adjust the semantics of the flag are probably
> constrained.

Well, there's no filesystem that would really support the DAX flag
currently (XFS accepts it, stores it, but does nothing with it). So I think
we are reasonably free in defining the semantics. Also I think this is
closely related to Dan's topic "What should be done to remove experimental
tag from DAX" (or however it was called).

							Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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