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

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Date:	Wed, 18 Aug 2010 11:32:20 -0700
From:	"Patrick J. LoPresti" <lopresti@...il.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"J. Bruce Fields" <bfields@...ldses.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 Wed, Aug 18, 2010 at 11:20 AM, David Woodhouse <dwmw2@...radead.org> wrote:
>
> Um, can't you? You can't *store* timestamps which are more precise, but
> they can be in cache can't they?

No.  That is how Linux used to work, and it caused many problems,
which is why the current_fs_time() function was invented.

> And since you're not going to drop it from cache and bring it back in
> again within 4ms, that ought to suffice?

Not the problem.  As usual, the problem is out-of-order timestamps:

1) Modify file A
2) Modify file B
3) File B's inode gets evicted, truncating its timestamp to disk resolution
4) Call stat() on B, bringing it back in with truncated resolution

And boom, B appears to be OLDER than A.  Which is not allowed.

This is exactly what happened when Linux first added sub-second
timestamps to the generic VFS layer.  Many complaints about "make"
rebuilding files unecessarily, among other things.  Eventually it got
fixed by the introduction of current_fs_time().

 - Pat
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