Mount only sub-directory in NFS export

Question

On my NAS I export one single filesystem:

/mnt/datastore  *(rw,sync,no_subtree_check,all_squash,fsid=0)

With /mnt/datastore/ being the mount point of a FUSE based concatenation of a bunch of disks.

A sub-directory in this export, /mnt/datastore/dev/ is where I store all the source trees for development work I'm busy with. I have a dedicated VM that I do development on and as such I want to mount this export on it. For safety purposes I am just interested in mounting the subdirectory mnt/datastore/dev/ so that when I mess up, I do not erase my entire collection of movies and TV series.

What I've tried:

I created a new export on the NAS:

/mnt/datastore/dev      *(rw,sync,no_subtree_check,all_squash,fsid=0)

and mounted that on the dev box in fstab with:

IP_OF_NAS:/mnt/datastore/dev /mnt/dev_directory    nfs     rsize=8912,wsize=8912,timeo=14,intr     0       0

This does not work as expected: It mounts fine, but I see the contents of /mnt/datastore/, not /mnt/datastore/dev/ which defeats the purpose of this exercise.

Solution

Replace no_subtree_check with subtree_check. Make sure fsid doesn't conflict with the parent export.

The subtree_check option tells the NFS server to take into account the path on the filesystem when checking its exports for any action. This is typically required when exporting directories along a tree sitting on a sole filesystem. Without this the NFS server will serve requests from the first export record matching the filesystem holding the path targeted by client's NFS commands.

Comments

[Xavier Lucas]

Replace no_subtree_check with subtree_check. Make sure fsid doesn't conflict with the parent export.

[getack]

Your trick worked. You can post it as a regular answer so that I can mark it as the answer. Can you also please explain why subtree checking is needed? I do not understand the documentation's description of it fully.

[Xavier Lucas]

Done. See below.

[pgoetz]

Using fsid=0 in both places is what the issue was. fsid=0 is reserved for the root of the NFS export hierarchy. It's not clear that subtree_check is necessary or desirable in this context.