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

From: Andy Lutomirski
Date: Wed Aug 27 2014 - 21:30:38 EST

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.

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