ZFS root set to readonly=on temporary at boot

[Xīcò]

Dear freebsd-fs,

As I am trying to install a new small server with a most partitions on
ZFS, I am running into some little read-only troubles. At boot the root
is always set to read-only. The rest of the system is fine though:

  # zfs get all | grep readonly
  zroot             readonly              on                     temporary
  zroot/tmp         readonly              off                    inherited from zroot
  zroot/usr         readonly              off                    inherited from zroot
  […]

I cannot seem to understand what may cause this issue. The system is a
typical manual [1] installation (used to work fine), on an empty system.

Note that I can reset the root to readonly=off with:

  # zfs set readonly=off zroot

but it is a pain to to it manually without understanding why. I also
cannot seem to get much info from the log messages:

  ==> /var/log/messages <==
  Feb 18 18:56:24  kernel: Root mount waiting for: usbus4
  Feb 18 18:56:24  last message repeated 2 times
  Feb 18 18:56:24  kernel: uhub4: 8 ports with 8 removable, self powered
  Feb 18 18:56:24  kernel: Root mount waiting for: usbus4
  Feb 18 18:56:24  kernel: ugen4.2: <vendor 0x058f> at usbus4
  Feb 18 18:56:24  kernel: uhub5: <vendor 0x058f product 0x6254, class 9/0, rev 2.00/1.00, addr 2> on usbus4
  Feb 18 18:56:24  kernel: uhub5: 4 ports with 4 removable, self powered
  Feb 18 18:56:24  kernel: Trying to mount root from zfs:zroot [rw]...
  Feb 18 18:56:29  kernel: .
  Feb 18 18:56:30  kernel: .
  
There are a couple of mentions to this same problem in 2012 on this
mailing list [2] and on an unanswered forum message [3], but they did
not solve the issue (setting vfs.root.mountfrom.options=rw only changes
the log message). The machine starts its other services (such as SSH)
correctly though.

Hope someone will have an idea.

Best,

Xīcò

[1] https://blog.gegeweb.org/article30/freebsd-9-2-en-full-zfs-sur-un-kimsufi-ps-4g
[2] https://lists.freebsd.org/pipermail/freebsd-fs/2012-February/013654.html
[3] https://forums.freebsd.org/threads/zfs-readonly-on-temporary-on-boot.28944/