zdb is very handy for diagnosing problems with a pool in a safe and
quick way. When a pool is in a bad shape, we often want to disable some
fail-safes, or adjust some tunables in order to open them. In the
kernel, this is done by changing public variables in mdb. The goal of
this feature is to add the same capability to zdb and ztest, so that
they can change libzpool tuneables from the command line.