In short, the fsid is the way client and server identify an export after it's mounted.
As the man page states, the fsid will be derived from the underlying filesystem, if not specified.
The four exports have the same fsid, so it's possible that when client1 is asking about the files from its mount, the server thinks it's trying to access client4's export (assuming it's keeping the latest occurrence of the same fsid only.)
I guess there are a few ways to validate this hypothesis, for instance by checking that one (and only one) of the 4 clients will work. Also by keeping only the client1 export, without the other 3, and confirming client1 will work then.
See also this answer for a way to query the fsid from a client, using the mountpoint -d command, which you could use from the 4 clients to confirm the 4 mounts have the same fsid.
Why is that a solution?
Because with distinct fsid's, the exports will look distinct to the NFS server, so it will properly match client accesses to their corresponding mounts.
Should we use fsid on every export?
Yes, I think that's a good practice, it ensures you'll keep the control and changes in the underlying storage devices and exports will not affect your clients.
(In my case, I recall adopting it since some of my NFS servers with disks on a SAN would sometimes scan disks in a different order, so after a reboot /dev/sdh would suddenly become /dev/sdj. Mounting using labels ensured it would be mounted at the correct location, but the fsid would change and clients would get lost. This is before the ubiquity of UUIDs, which apparently are now supported and are of course a much better solution for this, that wouldn't break when disks are scanned in a different order. But, still, specifying the fsid explicitly is not a bad idea, lets you keep full control.)
Best Answer
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