== ZFS can apparently start NFS fileservice before boot finishes

Here's something that I was surprised to discover [[the other day
ZFSNFSPoolQuotaProblem]]: ~~ZFS can start serving things over NFS
before the system is fully up~~. Unfortunately this can have a bad
effect because it's possible for this NFS traffic to cause further
ZFS traffic in [[some circumstances ZFSNFSPoolQuotaProblem]].

Since this sounds unbelievable, let me report what I saw first. As
our problem [[NFS fileserver ZFSFileserverSetupII]] rebooted, it
stalled reporting '_Reading ZFS config:_'. At the same time, our
iSCSI backends reported a high ongoing write volume to one pool's
set of disks and _snoop_ on the fileserver could see active NFS
traffic. _ptree_ reported that what was running at the time was the
'_zfs mount -a_' that is part of the /system/filesystem/local target.

(I recovered the fileserver from this problem by the simple method
of disconnecting its network interface. This caused _nlockmgr_ to
fail to start, but at least the system was up. ZFS commands like
'_zfs list_' stalled during this stage; I didn't think to do a _df_
to capture the actual mounts.)

Although I can't prove it from the source code, I have to assume
that '_zfs mount -a_' is enabling NFS access to filesystems as it
mounts them. An alternate explanation is that _/etc/dfs/sharetab_
had listings for all of the filesystems (ZFS adds them as part of
sharing them over NFS) and this activated NFS service for filesystems
as they appeared. The net effect is about the same.

This is obviously a real issue if you want your system to be fully
up and running okay before any NFS fileservice starts. Since
apparently some sorts of NFS traffic under some circumstances can
stall further ZFS activity, well, this is something you may care
about; we certainly do now.

In theory the SMF dependencies say that /network/nfs/server depends
on /system/filesystem/local, as well as nlockmgr (which didn't
start). In practice, well, how the system actually behaves is the
ultimate proof and all I can do is report what I saw. Yes, this is
frustrating. That ZFS and SMF together hide so much in black magic
is a serious problem [[that has made me frustrated before
ZFSPoolActivationII]]. Among other things it means that when something
goes odd or wrong you need to be a deep expert to understand what's
going on.