linux-kernel - Re: Proposal: Use hi-res clock for file timestamps

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Message-ID: <AANLkTi=w1UA5ZZDBigpxMiL7A7DnbnQhLkg62JZpC6Ri@mail.gmail.com>
Date:	Tue, 17 Aug 2010 12:18:12 -0700
From:	"Patrick J. LoPresti" <lopresti@...il.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Proposal: Use hi-res clock for file timestamps

On Tue, Aug 17, 2010 at 12:04 PM, J. Bruce Fields <bfields@...ldses.org> wrote:
>
> Right, I think that we probably have to give up ext3 as a lost cause.
> But perhaps we could get away with a hack like this on filesystems that
> can store nanoseconds.

I do not think so.

The problem with "increment mtime by a nanosecond when necessary" is
that timestamps can wind up out of order.  As in:

1) Do a bunch of operations on file A
2) Do one operation on file B

Imagine each operation on A incrementing its timestamp by a nanosecond
"just because".  If all of these operations happen in less than 4 ms,
you can wind up with the timestamp on B being EARLIER than the
timestamp on A.  That is a big no-no (think "make" or anything else
relying on timestamps for relative times).

If you can prove that the last modification on B happens after the
last modification on A, then it is very bad for the mtime on B to be
earlier than the mtime on A.  I guarantee that will break things in
the real world.

As you say, high-resolution timestamps "will extend the useful
lifetime of NFSv3 by quite a bit".  They are also a good idea in
principle, IMO.  Correctness is almost always more important than
performance.

 - Pat
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