BIOS limitations on size of bootable zpool?

Matthew Seaman m.seaman at
Thu Oct 21 12:01:41 UTC 2010

Dear all,

I'm happy that gptzfsloader will work with just about any zpool
configuration you could imagine, but...

We have an HP DL185 G5 with a P400 raid array, fully populated with 12
drives.  Since there's no JBOD mode (or at least, not one you can get to
from the BIOS configuration screens), the array is configured as 12
single disk RAID0 arrays.   As I posted about previously, we had FreeBSD
8.1-STABLE installed on a 6 disk raidz1, and everything was happy.
However, we were having some difficulty adding a second vdev -- another
raidz1 using the other 6 drives.

Well, to cut a long story short: eventually we did this by hot-plugging
disks 7 -- 12 after FreeBSD was up and running.  Everything was cool and
dandy, and we had the server running on all drives after setting up gpt
partition tables and doing a 'zpool add'.

Until we tested rebooting.

On attempted reboot, the loader reported 8 drives, and subsequently ZFS
flailed with the dreaded "ZFS: i/o error - all block copies unavailable"
error.  Now, we've had a poke through FreeBSD sources, and as far as we
can tell, FreeBSD will work with up to 31 devices being reported from
the BIOS.  Is this correct, and the limitation is in what the hardware
is reporting to the loader at the early stages of booting?

Any good tricks for getting round this sort of limitation?  Our current
plan is to set up a USB memstick with /boot on it, by adapting the
instructions here: -- which
isn't ideal as the memstick will be a single point of failure.



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
JID: matthew at               Kent, CT11 9PW

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