linux-kernel - Re: [LSF/MM TOPIC] The end of the DAX experiment

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Date:   Mon, 18 Feb 2019 10:17:30 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ric Wheeler <ricwheeler@...il.com>
Cc:     lsf-pc@...ts.linux-foundation.org,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [LSF/MM TOPIC] The end of the DAX experiment

On Sun, Feb 17, 2019 at 12:29 PM Ric Wheeler <ricwheeler@...il.com> wrote:
>
> On 2/6/19 4:12 PM, Dan Williams wrote:
> > Before people get too excited this isn't a proposal to kill DAX. The
> > topic proposal is a discussion to resolve lingering open questions
> > that currently motivate ext4 and xfs to scream "EXPERIMENTAL" when the
> > current DAX facilities are enabled. The are 2 primary concerns to
> > resolve. Enumerate the remaining features/fixes, and identify a path
> > to implement it all without regressing any existing application use
> > cases.
> >
> > An enumeration of remaining projects follows, please expand this list
> > if I missed something:
> >
> > * "DAX" has no specific meaning by itself, users have 2 use cases for
> > "DAX" capabilities: userspace cache management via MAP_SYNC, and page
> > cache avoidance where the latter aspect of DAX has no current api to
> > discover / use it. The project is to supplement MAP_SYNC with a
> > MAP_DIRECT facility and MADV_SYNC / MADV_DIRECT to indicate the same
> > dynamically via madvise. Similar to O_DIRECT, MAP_DIRECT would be an
> > application hint to avoid / minimiize page cache usage, but no strict
> > guarantee like what MAP_SYNC provides.
>
>
> Sounds like a great topic to me. Having just gone through a new round of USENIX
> paper reviews, it is interesting to see how many academic systems are being
> pitched in this space (and most of them don't mention the kernel based xfs/ext4
> with dax).

Makes sense, the current fsdax facility is chasing feature
deficiencies relative to device-dax (longterm pin support) and nova
(dax techniques for fs-metadata).