2.6 Kernel with inotify patch

Nick Hudson nhudson at lunar-linux.org
Tue Oct 26 15:51:10 UTC 2004

Hash: SHA1
I have made a linux-2.6 kernel with the inotify patch just for my
testing atm, but if anyone wants it you can get it at
http://nhudson.homelinux.org/files/linux-2.6.tar.bz2  .   My server is
down atm but it will be back up later around 12:00 cst.  if anyone
wants to try it.  In case you are wondering what inotify is here ya go:

Why Not dnotify and Why inotify

Everyone seems quick to deride the blunder known as "dnotify" and
applaud a
replacement, any replacement, man anything but that current mess, but
in the
name of fairness I present my treatise on why dnotify is what one
might call
not good:

* dnotify requires the opening of one fd per each directory that you
intend to
~  watch.
~    o The file descriptor pins the directory, disallowing the backing
~      device to be unmounted, which absolutely wrecks havoc with
~      media.
~    o Watching many directories results in many open file descriptors,
~      possibly hitting a per-process fd limit.
* dnotify is directory-based. You only learn about changes to directories.
~  Sure, a change to a file in a directory affects the directory, but
you are
~  then forced to keep a cache of stat structures around to compare
things in
~  order to find out which file.
* dnotify's interface to user-space is awful.
~    o dnotify uses signals to communicate with user-space.
~    o Specifically, dnotify uses SIGIO.
~    o But then you can pick a different signal! So by "signals," I really
~      meant you need to use real-time signals if you want to queue the
~      events.
* dnotify basically ignores any problems that would arise in the VFS
from hard
~  links.
* Rumor is that the "d" in "dnotify" does not stand for "directory"
but for
~  "suck."

A suitable replacement is "inotify." And now, my tract on what inotify
to the table:

* inotify's interface is a device node, not SIGIO.
~    o You open only a single fd, to the device node. No more pinning
~      directories or opening a million file descriptors.
~    o Usage is nice: open the device, issue simple commands via ioctl(),
~      and then block on the device. It returns events when, well,
there are
~      events to be returned.
~    o You can select() on the device node and so it integrates with main
~      loops like coffee mixed with vanilla milkshake.
* inotify has an event that says "the filesystem that the item you were
~  watching is on was unmounted" (this is particularly cool).
* inotify can watch directories or files.
* The "i" in inotify does not stand for "suck" but for "inode" -- the
~  choice since inotify is inode-based.

I will have a inotify-utils module ready by tonight I hope.

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