linux-kernel - Re: Beyond inotify recursive watches

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Date:	Fri, 5 Apr 2013 17:12:29 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jan Kara <jack@...e.cz>
Cc:	Ramkumar Ramachandra <artagnon@...il.com>,
	linux-kernel@...r.kernel.org, Junio C Hamano <gitster@...ox.com>,
	Thomas Rast <trast@....ethz.ch>,
	Duy Nguy???n <pclouds@...il.com>, Jeff King <peff@...f.net>,
	Karsten Blees <karsten.blees@...il.com>
Subject: Re: Beyond inotify recursive watches

On Fri, Apr 05, 2013 at 05:55:34PM +0200, Jan Kara wrote:

> What your question reminds me is an idea of recursive modification time
> stamp on directories. That is a time stamp that gets updated whenever
> anything in the tree under the directory changes. Now this would be too
> expensive to maintain so there's also a trick implemented that you update
> the time stamp (and continue updating recursive time stamps upwards) only
> if a special flag is set on the directory. And you clear the flag at that
> moment. So until someone checks the time stamp and resets the flag no
> further updates of the recursive modification time happen.
> 
> This scheme works for arbitrary number of processes interested in recursive
> time stamps (only updates of the time stamps get more frequent). What is
> somewhat inconvenient is that this only tells you something in the
> directory or its subtree changed so you still have to scan all the
> directories on the path to modified file. So I'm not sure of how much use
> this would be to you.

Feel free to write up the details of locking you'll need for that.  It will
*not* be fun...
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