While inotifywait is commonly used in combination with rsync to backup/synchronize data on a quasi-real-time basis, it may not work properly on NAS file systems due to the implementation of inotify.
How inotify works
inotify is a sub-module of the Linux kernel, and inotifywait is the user-mode interface of inotify. inotify is realized at the VFS layer. When file operations reach the VFS layer, the inotify module sends the operation type (creation/deletion/attribute change, and so on) and operation object (file name) to the user-mode, and the user-mode inotifywait then outputs the operation information to the user.
Because inotify is implemented at the VFS layer of the kernel, the local kernel cannot recognize operations made by a remote client on the NFS file system. Therefore, inotify cannot recognize modifications on files made by the remote client.
- inotifywait recognizes operations on files in the mounted directory on Client A.
- inotify cannot recognize any operation on files in the mounted directory on Client B.
An alternative solution is to use FAM.
FAM is a library used for monitoring files or directories, and it is fully implemented in user-mode. You then only need to run a daemon in the background to regularly scan the directory and check for file changes.
- You must write a program to call the FAM interface implementation function.
- In scenarios with a large number of files, using FAM may have poor performance and consume a lot of resources. Furthermore, it cannot ensure real-time monitoring.