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

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Message-ID: <20100817203941.729830b7@lxorguk.ukuu.org.uk>
Date:	Tue, 17 Aug 2010 20:39:41 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"Patrick J. LoPresti" <lopresti@...il.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	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

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

Surely that depends on your implementation ?

> 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).


[time resolution bits of data][value incremented value for that time]


	if (time_now == time_last)
		return { time_last , ++ct };
	else {
		ct = 0;
		time_last = time_now
		return { time_last , 0 };
	}

providing it is done with the same 'ct' across the fs and you can't do
enough ops/second to wrap the nanosecs - which should be fine for now,
your ordering is still safe is it not ?

> 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.

Alan
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