Comment by jjav

Thanks, I see. It feels as if you're describing using a mirror pool as something that it isn't so I get the impedance mismatch.

> taking them offline ‘forever’ (really over multiple boot cycles) it makes the running ZFS instance angry because it expects those mirror copies to still be accessible somewhere

If a drive dies that's normal, it can be removed from the pool.

> Also, if you reconnect those old instances, it will try to resilver the older copies (and hence modify them).

I mean yes, because that's what one would want if it is a mirror. Every device should contain the same data so if one drive comes back from the dead it should be resilvered to be up to date.

If what you need is archival as opposed to a mirror pool, I'd say use snapshot and send to preserve that point in time in a backup somewhere.