linux-kernel - Re: [RFC 1/2] fanotify: new event FAN_MODIFY

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Date:   Tue, 14 Mar 2017 13:41:52 +0100
From:   Filip Štědronský <r.lkml@...narg.cz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC 1/2] fanotify: new event FAN_MODIFY_DIR

Hi,

On Tue, Mar 14, 2017 at 12:11:40PM +0200, Amir Goldstein wrote:
> >   - file system indexers / desktop search tools
> >   - file synchronization tools (like Dropbox, Nextcloud, etc.),
> >     online backup tools
> 
> This last one is the use case of my employer, Ctera Networks.
> Out of curiosity, what is the use case that you are focusing on?

I'm working on a file synchronization tool as part of my bachelor
thesis at Charles University.

When I started (now over a year ago ... long story), there were AFIK no
attempts at solving the recursive inotify issue, only a lot of
complaints, so I cobbled up together something simple (I'm not a kernel
developer by trade, this was my first patch) that would allow me to
work on the userspace parts, which are the main bulk.

I try to focus on algorithmic and implementation efficiency as opposed
to fancy GUIs and similar. I want it to be fast with 100k directories
and 10M files in home dir. But it's very WIP so we'll see how that turns
out.

> I had the feeling that all recursive inotify users are hiding in the shadows,
> but was missing more concrete evidence.

Yes, even Dropbox uses inotify. They have articles in their help on how
to increase inotify.max_user_watches: https://www.dropbox.com/help/145.
That's not good PR. ;-) (I'm not affiliated with DB, just pointing that
out.)

> About the argument of not having to change in-kernel framework,
> I don't think it should be a consideration at all.

Understood. I tried to stay conservative and non-controversial because I
imagined that radical framework changes would take months of discussions
(look at the history of statx) and this issue seems to be pressing for
quite a lot of people.  But rushing is of course not the best strategy
either, there are always compromises.

> If you don't specify FAN_EVENT_INFO_NAME, you can get filename events
> FAN_MOVE|FAN_CREATE|FAN_DELETE without the name.
> 
> What you do get is the file descriptor of the parent. sounds familiar? ;-)

I didn't notice this bit. That sounds like a win-win.

Filip