Re: Issues with using fanotify for a filesystem indexer

[Alexander Larsson <alexl@redhat.com>]

On Thu, 2009-04-02 at 18:50 +0200, Jan Kara wrote:
> On Thu 02-04-09 18:29:04, Alexander Larsson wrote:
> > Another potential issue with this is that every change bubbles up to the
> > top, modifying the recursive mtime of that. This will become very
> > contented, and may imply a partial serialization of fs activity, which
> > is kinda costly.
>   Not every change - only the first change bubbles to the top, clearing the
> flag on its way. Then next change stops bubbling up as soon as it reaches
> a directory with the flag cleared. So no contention happen - we update flag
> + timestamp only at most once per scan of the directory by indexer (or
> someone else interested in recursive mtime) => once per a few minutes on
> average system.

Ah, I see. The indexer sets the flag. 
I see some issues. First of all, writing the flag/mtime to disk seems
like a bad idea. It'll cause a lot of writing when the indexer recurses
throught the filesystem, similar to atimes. But, if you're not
persisting the flag/mtime then you need to keep all the dentries with
the flag set in memory, which has resource use risks similar to
unbounded event queues.