> > I'm hoping to be able to switch an existing
> > VirtualBox vdi from IDE to SATA (for
> > the likely performance boost).  But the existing
> boot
> > archive can't handle that,
> > since it only knows about root being on the IDE.
> 
> ZFS boot / root?
> 
> All that is required is to update the physical device
> path
> in the zpool label on disk.  The label is
> automatically
> updated when the pool gets imported.  The following
> should work:
> 
> - switch from P-ATA to S-ATA
> 
> - boot from the live cd into single user mode,
>   zpool import -f rpool
> (this updates the bootpath in the zpool's label)
> 
> - reboot from the HDD

Thanks - that's good to know for the details.

I'd just finished doing it with the zfs  mirror approach though.  In the 
process,
I did run into one thing that might have been a problem trying to reuse the
existing VDI: SATA seems to leave one less cylinder available for use than IDE;
I ended up making the VDI for the SATA a little larger (which presumably
becomes available when any smaller mirrors are detached; so it's not wasted).

Adding the new VDI and making it into a zfs mirror is pretty straightforward, 
if slow.
The only thing I hadn't anticipated was that it's "installgrub", not 
"installboot" (most
of my prior experience is SPARC...).

There's definitely some performance impact - the host CPU graph I have running
is visibly staying lower under similar usage now.  But I didn't attempt any 
measurements,
so I can't put a number on that (although I think others have suggested that it 
could
result in as much as a reduction of 20% of total host CPU).
-- 
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