* [LSF/MM TOPIC] Standardizing semantics around the per-file DAX flag
@ 2019-02-13 16:40 Theodore Y. Ts'o
2019-02-19 12:35 ` Jan Kara
0 siblings, 1 reply; 2+ messages in thread
From: Theodore Y. Ts'o @ 2019-02-13 16:40 UTC (permalink / raw)
To: lsf-pc; +Cc: linux-fsdevel, linux-nvdimm
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
^ permalink raw reply [flat|nested] 2+ messages in thread* Re: [LSF/MM TOPIC] Standardizing semantics around the per-file DAX flag
2019-02-13 16:40 [LSF/MM TOPIC] Standardizing semantics around the per-file DAX flag Theodore Y. Ts'o
@ 2019-02-19 12:35 ` Jan Kara
0 siblings, 0 replies; 2+ messages in thread
From: Jan Kara @ 2019-02-19 12:35 UTC (permalink / raw)
To: Theodore Y. Ts'o; +Cc: lsf-pc, linux-fsdevel, linux-nvdimm
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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