Linux's NFS export handling system has a very convenient option where you don't have to put all of your exports into one file, /etc/exports, but can instead write them into a bunch of separate files in /etc/exports.d. This is very convenient for allowing you to manage filesystem exports separately from each other and to add, remove, or modify only a single filesystem's exports. Also, one of the things that exportfs(8) can do is 'reexport' all current exports, synchronizing the system state to what is in /etc/exports and /etc/exports.d; this is 'exportfs -r', and is a handy thing to do after you've done various manipulations of files in /etc/exports.d.
Although it's not documented and not explicit in 'exportfs -v -r' (which will claim to be 'exporting ...' for various things), I have an important safety tip which I discovered today: exportfs does nothing on a re-export if you have any problems in your exports . In particular, if any single file in /etc/exports.d has a problem, no files from /etc/exports.d get processed and no exports are updated.
One potential problem with such files is syntax errors, which is fair enough as a 'problem'. But another problem is that they refer to directories that don't exist, for example because you have lingering exports for a ZFS pool that you've temporarily exported (which deletes the directories that the pool's filesystems may have previously been mounted on). A missing directory is an error even if the exportfs options include 'mountpoint', which only does the export if the directory is a mount point.
When I stubbed my toe on this I was surprised. What I'd vaguely expected was that the error would cause only the particular file in /etc/exports.d to not be processed, and that it wouldn't be a fatal error for the entire process. Exportfs itself prints no notices about this being a fatal problem, and it will happily continue to process other files in /etc/exports.d (as you can see with 'exportfs -v -r' with the right ordering of where the problem file is) and claim to be exporting them.
Oh well, now I know and hopefully it will stick .