I've recently created a dropbox system using inotify, watching for files created in a particular directory. The directory I'm watching is mounted from an NFS server, and inotify is behaving differently than I'd expect. Consider the following scenario in which an inotify script is run on machine A, watching /some/nfs/dir/also/visible/to/B.
-Using machine A to create a file in /some/nfs/dir/also/visible/to/B, the script behaves as expected. Using machine B to carry out the same action, the script is not notified about a new file dropped in the directory.
-When the script is run on the NFS server, it gets notified when files are created from both machine A and machine B.
Is this a bug in the bug in the package I'm using to access inotofy, or is this expected behaviour?
Answer
inotify requires support from the kernel to work. When an application tracks a directory, it asks the kernel to inform it when those changes occur. When the change occurs, in addition to writing those changes to disk, the kernel also notifies the watching process.
On a remote NFS machine, the change is not visible to the kernel; it happens entirely remotely. NFS predates inotify and there is no network level support for it in NFS, or anything equivalent.
If you want to get around this, You can run a service on the storage server (since that kernel will always see changes to the filesystem) that brokers inotify requests for remote machines, and forward the data to the remote clients.
Edit: It seems odd to me that NFS should be blamed for its lack of support for inotify.
Network File System (NFS) is a distributed file system protocol originally developed by Sun Microsystems in 1984, wikipedia article
However:
Inotify (inode notify) is a Linux kernel subsystem that acts to extend filesystems to notice changes to the filesystem. [...] It has been included in the mainline Linux kernel from release 2.6.13 (June 18, 2005 ) [...]. wikipedia article
It's hard to expect a portable network protocol/application to support a specific kernel feature developed for a different operating system and that appeared more than twenty years later. Even if it did include extensions for it, they would not be available or useful on other operating systems.
*emphasis mine in all cases
Another problem with this; Lets suppose we are not using a network at all, but rather, a local filesystem with good inotify support: ext3 (suppose its mounted at /mnt/foo). But instead of a real disk, the filesystem is mounted from a loopback device ; and the underlying file is in turn accessible at a different location in the vfs (say, /var/images/foo.img).
Now, you're not supposed to modify mounted ext3 filesystems, But it's still reasonably safe to do so if the change is to file contents instead of metadata.
So suppose a clever user modifies the file system image (/var/images/foo.img) in a hex editor, replacing a file's contents with some other data, while at the same time an inotify watch is observing the same file on the mounted filesystem.
There's no reasonable way one can arrange for inotify to always inform the watching process of this sort of change. Although there are probably some gyrations that could be take to make ext3 notice and honor the change, none of that would apply to, say, the xfs drtiver, which is otherwise quite similar.
Nor should it. You're cheating!. inotify can only inform you of changes that occured through the vfs at the actual mountpoint being watched. If the changes occured outside that VFS, because of a change to the underlying data, inotify can't help you and isn't designed to solve that problem.
Have you considered using a message queue for network notification?