I read the "high availability boot pool" thing, and don't really understand the point.
The point is that your PC BIOS will not boot from the secondary device in a ZFS setup if the primary device is only partially failed. For example, if you have two El Cheapo 60GB SSD's and the one on SATA 0 develops an inability to read the boot LBA's, your BIOS will nevertheless see that SATA 0 is present and try to boot from it, fail, and hang with the dreaded "Disk Read Error" message or whatever your particular BIOS does.
A HBA in IR mode allows the HBA to notice that it is getting a read error on SATA 0 LBA 0, and will fail over to SATA 1 LBA 0, and present that data to the host, thereby allowing bootup.
This does not completely eliminate every possible impediment to booting, but it does get rid of a major one.
running on top of RAID when pretty much everywhere ZFS specifically recommends NOT running on top of RAID doesn't make any sense to me,
The reasons not to run on RAID are explained in my article
1) An HBA is a Host Bus Adapter. This is a controller that allows SAS and SATA devices to be attached to, and communicate directly with, a server. RAID controllers typically aggregate several disks into a Virtual Disk abstraction of some sort...
www.truenas.com
and as it turns out, the most devastating reasons not to use a RAID controller do not apply to an HBA in IR mode. There's no cache, there's no battery, there IS a highly ZFS-compatible driver, etc. The IR mode does hide the state of the component disks from ZFS, and also hides the redundancy, which is why you might do some sas2ircu scripting to check the state of the boot device, and also why you pair it with a third SSD to provide the ZFS-level redundancy.
if you are an advanced TrueNASer, and you have your backups and understand the risks, sure, put the whole pool on RAID,
Nobody's advocating such an idiotic thing as running your data pools from RAID. The highly available boot pool thing is a very special case that is only intended to handle the bootstrap from BIOS issue.