I currently have my boot pool set up as a mirror, and out of sheer paranoia I thought I might switch it to be a 3x mirror…you never know if the second SSD will fail while remirroring ~5 GB!
Am I crazy, or is there no way to do this in the GUI? I only have the option to replace the existing drives. Is it possible to at least set a drive as a spare for the boot pool?
Used to be in Core at least. No longer, and also doesn’t work via middleware cli.
Huh. Any particular reason?
I don’t know of a reason.
But if I had to guess I’d imagine that there was an issue with grub, or exponential complexity as more devices are added.
Or maybe there’s no good reason.
Even if it does fail, just reinstall on a new drive, import the config file, and voila!
I mean yes, double boot pool drive failure is hardly an ultra disaster scenario, but surely if the option were available it would be preferable to avoid the situation entirely.
There are other uses too
For example, a third of the mirror could be a virtual machine hypervisor disk and the other two members could be low grade usb drives allowing booting into virtualized and bare metal TrueNAS installs 
Or, perhaps as part of an experimental in-place core to scale migration
Add a 3rd mirror, then migrate that.
Wow, you just introduced me to a completely cursed strategy for using mirrors 0_0. I am absolutely terrified by the fact that the problem-solving part of my brain now has access to this paradigm.
You’re welcome
Except 3-way mirroring is no longer accessible, remembering that it’s not just zfs mirroring but the boot partitions etc too.
Hmm, yeah that makes sense. But it seems that assigning spares should at least still be viable.
FWIW, boot pool mirroring doesn’t magically mean that your BIOS selects the new surviving disk to boot from.
That’s where IPMI comes in though, so that you can dial in to your VPN from the ISS and select the other boot disk
Consider this instead:
