<b>No subtree check</b>
<p>
This option disables subtree checking, which has mild security implications, but can improve reliability is some circumstances.
</p>
<p>
If a subdirectory of a filesystem is exported, but the whole filesystem is not, then whenever a NFS request arrives, the server must check not only that the accessed file is in the appropriate filesystem (which is easy) but also that it is in the exported tree (which is harder). This check is called the subtree_check.
</p>
<p>
In order to perform this check, the server must include some information about the location of the file in the "filehandle" that is given to the client. This can cause problems with accessing files that are renamed while a client has them open (though in many simple cases it will still work).
</p>
<p>
subtree checking is also used to make sure that files inside directories to which only root has access can only be accessed if the filesystem is exported with no_root_squash (see below), even the file itself allows more general access.
</p>
<p>
As a general guide, a home directory filesystem, which is normally exported at the root and may see lots of file renames, should be exported with subtree checking disabled. A filesystem which is mostly read-only, and at least does not see many file renames (e.g. /usr or /var) and for which subdirectories may be exported, should probably be exported with subtree checks enabled.
</p>
