linux-kernel - Re: [PATCH v10 00/21] Support ext4 on NV-DIMMs

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Message-ID: <53FE8633.10305@amacapital.net>
Date:	Wed, 27 Aug 2014 18:30:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...ux.intel.com>
CC:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 00/21] Support ext4 on NV-DIMMs

On 08/27/2014 02:46 PM, Andrew Morton wrote:
> I assume (because I wasn't told!) that there are two objectives here:
> 
> 1) reduce memory consumption by not maintaining pagecache and
> 2) reduce CPU cost by avoiding the double-copies.
> 
> These things are pretty easily quantified.  And really they must be
> quantified as part of the developer testing, because if you find
> they've worsened then holy cow, what went wrong.
> 

There are two more huge ones:

3) Writes via mmap are immediately durable (or at least they're durable
after a *very* lightweight flush).

4) No page faults ever once a page is writable (I hope -- I'm not sure
whether this series actually achieves that goal).

A note on #3: there is ongoing work to enable write-through memory for
things like this.  Once that's done, then writes via mmap might actually
be synchronously durable, depending on chipset details.

--Andy
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