Re: filesystem change monitoring

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

On Tue, Aug 11, 2015 at 02:44:10PM +0000, Emmanuel Dreyfus wrote:
> Do we have a smart way to monitor a whole filesystem hierarchy for 
> changes? 

Replying to myself: the short answer is no. kqueue will not scale to 
thousands of nodes because it needs to open a filedescriptor for each.

lsyncd has been mentionned in the thread. On NetBSD, it uses kqueue
and therefore suffers the sclaing limitation.

An ongoing project to fix kqueue was mentionned:
https://wiki.netbsd.org/projects/project/kqueue-dirnotify/

That situation begs for an interm solution. Since my problem is to mirror
a file hierarchy from an host to another, I crafter an utility called
fsssyncd as an attempt to address the problem without lenghlty kernel 
hacking (that will be necessary, but not this week!):
http://ftp.espci.fr/pub/syncffsd/syncffsd-0.1.tgz

Basic usage:
syncfssd /usr/local/source host:/usr/local/destination

From the man page:
> syncfssd uses rsync(1) to replicate a directory tree to another host.
> This involves detecting changes, something NetBSD is not well prepared to
> handle. The kqueue(2) interface can detect changes, but it needs to use a
> file descriptor for every node monitored, and this does not scales very
> well for huge directories trees.
>
> syncfssd attempts to improve the situation until kqueue(2) gets better,
> by using FFS superblock's fs_time field, which is updated on every
> filesystem change. This lets syncfssd detects a change immediatly if the
> filesystem is mounted synchronous, or after a few seconds if mounted with
> WAPBL(4) enabled.
>
> Once a change is detected, the relevant nodes are found by walking source
> looking for files that changed since the previous filesystem modifica-
> tion.  The resulting nodes are deduped (i.e.: we do not retain a node in
> a directory if the directory itself changed), and are fed to rsync(1).

This logic could be embedded into lsyncd but I am not sure such work makes
sense. Improving kqueue seems a better, more general goal.

Feedbacks on this version 0.1 are welcome.

--
Emmanuel Dreyfus
manu%netbsd.org@localhost