FreeBSD ZFS v13 MFC — force unmount is experimental — finale

EDIT: And just like I predicted, my blog is gradually beginning to see more and more hits from search engine queries for “force unmount is experimental” or “zfs force unmount”.

I’ve talked to Kip about this. Below is the Email response I got this morning:

On Sun, May 24, 2009 at 1:26 AM, Jeremy Chadwick wrote:
> On Sun, May 24, 2009 at 12:58:19AM -0700, Kip Macy wrote:
>> On Sun, May 24, 2009 at 12:57 AM, Kip Macy wrote:
>> > Doing a forced unmount means that we try to force data out to disk
>> > before shutting down. Most of consider this desirable.
>>
>> Most of us consider this desirable.
>
> A few questions:
>
> 1) Isn't this already being done during the "Syncing disks, vnodes
> remaining..." stage of kernel shutdown?

You are watching a force unmount in progress when you see that.

> 2) Doesn't "zfs unmount -a" or "zfs volfini" (particularly the latter)
> do this for us?

Evidently not. The force unmount flag was passed.

> 3) If the forced unmount is intended/by design, is the ZFS_LOG() really
> necessary?  Is this something specific to the FreeBSD adaptation of ZFS?
> I haven't ever seen this message on our hundreds of Solaris 10 machines
> using ZFS at work, which is why it caught my eye.

Prior to my recent changes there was a similar message - force unmount
was disallowed entirely. Problem with that though is that people with
ZFS root lose data sometimes.

This is FreeBSD specific.

-Kip

So it seems the quirk (not the message itself, but the commit to permit force unmount) is FreeBSD-centric. The call to ZFS_LOG(), IMHO, is superfluous and should be removed. Otherwise I expect to see more and more hits on my blog for this issue once 7.3-RELEASE is made available…

This Email does not give me “warm fuzzies” in regards to ZFS being used on a root filesystem, which indirectly means booting ZFS directly is also impacted. Given that ZFS is incredibly resilient about data integrity, this quirk does not sit well with me, especially given that OpenSolaris doesn’t behave this way.

For now, I’d urge people to be very careful about ZFS for a root filesystem on FreeBSD. I know some folks have used it reliably for quite some time, but Kip knows more about the inner-workings than I do.