NFS mount to a subdirectory of already mounted directory

Question

We have a strange scenario (AIX 5.3) -

There is a directory /mnt to which an NFS share from windows server1 is mounted

Then there is another directory /mnt/subdir1 to which NFS share from windows server2 is mounted.

It is noticed that the second mount /mnt/subdir1 fails on system restarts. I can manually mount this location using mount command.

Is the failure due to nested nature of the /mnt directory structure?

In /etc/filesystems - entry for /mnt appears before /mnt/subdir1

Answer 1

mount (1) requires a mount point to exist in order to mount something. So, if /mnt/subdir1 doesn't exist an attempt to mount something there will fail.

I was not aware that you could mount over a mount point that is mounted from NFS, although you can do it with other file systems (e.g. you can mount /usr under / and /usr/local under /usr, so I guess that NFS is no exception.

However, I wouldn't recommend mounting under a mount point mounted via NFS anyway - you should really mount under two different subdirectories of /mnt or something off a local file system, e.g.

/mnt/subdir1
/mnt/subdir2

The obvious failure mode of mounting over a NFS mounted file system is that the server exporting the file system could go down. The OP's problem is not peculiar to NFS; the same issue would occur if other file systems were mounted in the wrong order.