linux-kernel - Re: [PATCH v8 1/1] f2fs: dax: implement direct access

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Message-ID: <20170726201136.GA30039@infradead.org>
Date:   Wed, 26 Jul 2017 13:11:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        sunqiuyang <sunqiuyang@...wei.com>
Subject: Re: [PATCH v8 1/1] f2fs: dax: implement direct access

On Wed, Jul 26, 2017 at 12:16:11PM -0700, Dan Williams wrote:
> Silently turn on DAX if HMAT says its ok?

Yes, absolutely.  I want my system to do the right thing by default,
and if HMAT says bypassing the page cache is a clear advatange it
should be the default.

> I think we would instead
> want a "-o autodax" for that case and then "-o dax" and "-o nodax" for
> the force cases.

Why?

> I think it's easier to administer than the dax mount option. If
> someone wants dax on only in a sub-tree they can set the flag on that
> parent directory and have a policy in dax filesystems that children
> inherit the dax policy from the parent. That seems a better
> administrative model than trying to get it all right globally at mount
> time.

And why exactly? If DAX is faster for file a in directory X it will
be just as fast for a file b in directory Y.