Heads-up: Possible regression between 10.0-RELEASE and 10.1-BETA1 with '/ on ZFS' setup

[Steven Hartland]

----- Original Message ----- 
From: "Glen Barber" <gjb at FreeBSD.org>
> During testing of the 10.1-RC1, I set up a multi-disk VirtualBox VM to
> test '/ on ZFS' in various scenarios.  FreeBSD 10.1-RC1 i386, when
> installed on ZFS with more than one disk (i.e., mirror, raidz-1,
> raidz-2, etc.) crashes when rebooting post-install.
> 
> This does not happen with a single-drive '/ on ZFS' setup under the same
> configuration.
> 
> FreeBSD 10.0-RELEASE i386 does not exhibit this behavior, however
> upgrading from 10.0-RELEASE to 10.0-BETA1 or later may exhibit
> a double-fault panic on boot.
> 
> A possible solution to this is to set kern.kstack_pages=4 via loader(8),
> however in my tests (solely in VirtualBox), this has been ineffective.
> 
> It is unclear to me right now if this is something specific to
> VirtualBox or not, though this problem was reported recently through
> Bugzilla ( https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194015 ).
> 
> I am still bisecting the stable/10 tree to try to identify when this
> possible regression was introduced, in addition to scrounging up the
> hardware to be able to test this on a bare-metal machine to determine if
> this is a VirtualBox (or hypervisor in general) specific problem, but
> this is taking longer than initially expected.
> 
> To be perfectly clear, the panic does occur in my particular testing
> environment as far back as 10.1-BETA1, so this is *not* something new to
> the upcoming 10.1-RC1.
> 
> If you have a multi-drive '/ on ZFS' setup (mirror, raidz-1) *without*
> PAE, and have upgraded to 10.1-BETA1 or later, please speak up in case
> this is a problem specific to my testing environment, which will likely
> be at least another day before I can verify is the case.

This has been a known issue on i386 since the switch to Clang see UPDATING:
20121223:
        After switching to Clang as the default compiler some users of ZFS
        on i386 systems started to experience stack overflow kernel panics.
        Please consider using 'options KSTACK_PAGES=4' in such configurations.

In my experience your millage may vary but essentially without 4 stack pages
all bets are off in terms of stability.

    Regards
    Steve