mjhjmtu

Problem statement (note that this problem has been solved, but there is a question about why the solution works)

The NFS server is Ubuntu 16.04.4 LTS. The clients are a mix of Ubuntu 16.04.4 LTS and CentOS 6.10 and 7.

NFS server has been working fine for months, and one particular export was serving several clients for their backups. The NFS server directory looked like this:

/mnt/backups/client1
/mnt/backups/client2
/mnt/backups/client3
/mnt/backups/client4

The /etc/exports contained:

/mnt/backups 1.2.3.0/24(rw,sync,no_subtree_check)

The client only mounts the nfs server during a backup, and then unmounts the backup when it is done.

This was working fine, however, it was determined that the clients should not be able to see each other in the /mnt/backups dir. Each client is using the same backup uid/gid. Therefore, a decision was made to separate out the directories through the use of the /etc/exports file.

To that end, the NFS server was stopped, and the /etc/exports was modified so it contains:

/mnt/backups/client1 1.2.3.21(rw,sync,no_subtree_check)
/mnt/backups/client2 1.2.3.22(rw,sync,no_subtree_check)
/mnt/backups/client3 1.2.3.23(rw,sync,no_subtree_check)
/mnt/backups/client4 1.2.3.24(rw,sync,no_subtree_check)

Recall, the client only mounts the NFS server when it is doing a backup (at 4am). On the server the NFS service was restarted, and the exports are checked with exportfs, it looks good.

OK, testing client1:

mount nfserver:/mnt/backups/client1 /mnt/client1

works fine, however, any action on /mnt/client1 results in:

cannot open directory /mnt/client1/: Stale file handle

Actions taken to resolve (which did not work): Restarting NFS on server. Restarting client. lsof |grep /mnt on client and server to see if any programs were holding the files open. Permissions checks on server/client. Again, switching the NFS /etc/exports back to the old file and mounting the nfs server from the client works. Switching back to the "new" method does not work.

After much gnashing of teeth, man pages and STFW only to find answers like "restart NFS", I recalled I had this problem years ago, and for some reason fsid had something to do with the solution. After reading the man pages, the following was added to the NFS server /etc/exports file:

/mnt/backups/client1 1.2.3.21(fsid=101,rw,sync,no_subtree_check)
/mnt/backups/client2 1.2.3.22(fsid=102,rw,sync,no_subtree_check)
/mnt/backups/client3 1.2.3.23(fsid=103,rw,sync,no_subtree_check)
/mnt/backups/client4 1.2.3.24(fsid=104,rw,sync,no_subtree_check)

Again, after this action the only thing performed was an exportfs -ra on the server.

Now all clients can mount the nfs server exports and they all work.

Why is that a solution?

Should we use fsid on every export?

Reading a man page like this one does not seem to clearly explain why the fsid is a solution. I had an idea that it could be that the stale mount was some kind of NFS file handler on the client end (or perhaps the server side), but for that to persist after a reboot would seem strange.