When a ZX file tree operates by relying on another ZX tree deeper in the stack, it adapts its control interface to behave as a facade for it.
For example, the /Ctl file of a ZX file tree can be user to see the status of the debug flags, to see who is using the file tree (in the cases of trees served to the network) and to see usage statistics for the system.
This is the result of reading /Ctl on a stack made out of a CFS that uses a MFS as a cache, and a RFS as the remote tree. The RFS was connected to a remote server exporting a CFS that uses a MFS cache to serve a LFS. What CFS, MFS, RFS, and LFS means is not really relevant. They are just ZX file servers that rely on underlying file servers to do its job (but, CFS is a caching file sever, MFS is a RAM FS, RFS is a remote file server, and LFS is a local file server).
I have removed some of the lines when they do not help to further illustrate the case.
> cat /zx/Ctl
stat 8220 calls 252 errs 16440 msgs 0 bytes
bgn: min 4.911µs avg 625.628µs max 1.333559776s
end: min 5.045µs avg 625.722µs max 1.333559891s
get 3147 calls 0 errs 16501 msgs 38660497 bytes
bgn: min 54.389µs avg 2.657265ms max 856.808309ms
end: min 63.349µs avg 5.141675ms max 856.841591ms
stat 78449 calls 2721 errs 156898 msgs 0 bytes
user rfs:126.96.36.199:65348 nemo as nemo on 2015-03-26
user rfs:188.8.131.52:65349 nemo as nemo on 2015-03-26
stat 2656 calls 0 errs 5312 msgs 0 bytes
stat 47588 calls 2487 errs 95176 msgs 0 bytes
stat 2854 calls 0 errs 5708 msgs 0 bytes
Now, note the lines starting with cfs:, cmfs:, and mfs:lsub:. Only the first section comes from the top-level CFS. Remaining lines come from such server reading the /Ctl file of the underlying ZX file tree and appending it to the data served to the user; and so on as we go down on the stack.
Thus, the user may inspect the ZX stack as deep as it wants.
In the same way, we can set debug on for CFS by executing
> echo debug on > /zx/Ctl
The ZX file server understands writes to the control file as textual commands
to perform control requests.
But we can also set debug on the third tree in the stack:
> echo pass pass debug on > /zx/Ctl
When a control request starts with pass, this word is removed from the request
and the server writes what remains of the control requests to the next tree in the stack.
This makes the control interface for all the trees in the stack available to the final user.