One of the tiny irritations of operating ZFS for a long time is that eventually, running 'zpool status' on your pools would produce a multi-line nag about upgrading them to the latest version of ZFS. I assume that this was added to 'zpool status' output so that you wouldn't be unaware of it, but the size of the message was far too large for its actual importance. Back in the old days of Solaris 10, 'zpool status -x' even included pools that could be upgraded (this was one of our Solaris 10 update 6 gotchas), but fortunately people have gotten more sensible since then. Now it's only a multi-line message.

Perhaps you think I'm exaggerating. No, really, here is the message from the latest version of OpenZFS:

status: Some supported and requested features are not enabled on the pool.
        The pool can still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
        the pool may no longer be accessible by software that does not support
        the features. See zpool-features(7) for details.

That's five lines for what should be one line. When you have multiple pools on a system, as we do in our fileserver environment, it adds up fast.

There are various operational reasons why you might not want to upgrade pools right away. Historically we didn't want to upgrade pools until we were certain we were staying on the new OS and ZFS version, and then once we confident we were staying we weren't certain about the impact on our NFS servers. In theory pool upgrades should be transparent; in practice, who knows.

(Right now all of our fileserver pools are up to date in some sense, because they were freshly created on our current fileservers. But the ZFS version the fileservers are running is out of date, and when we upgrade them next year we'll run into this.)

Fortunately OpenZFS 2.1.0 provides a feature that lets you shut this up, in the form of OpenZFS's support for partial upgrades. If you set the new 'compatibility' property to what you already have, 'zpool status' won't nag you (although 'zpool upgrade -v' will show you what you're missing).