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

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Message-ID: <20170825151445.ycf5xomoxvebgaez@thunk.org>
Date:   Fri, 25 Aug 2017 11:14:45 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-ext4@...r.kernel.org, Lukas Czerner <lczerner@...hat.com>,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH] ext4: introduce per-inode DAX flag

On Fri, Aug 25, 2017 at 12:54:15AM -0700, Christoph Hellwig wrote:
> > 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.

Crash and burn meaning the *kernel* will crash and burn?  Or will data
be damaged?

Given that, maybe XFS should withdraw support for the DAX and hope no
one is actually using it?

					- Ted