freebsd stable - Feb 2010 - booting off a ZFS pool consisting of multiple striped mirror vdevs

Hello

I have succesfully tested and used a "full ZFS install" of FreeBSD 8.0
on both single disk and mirror disk configurations using both MBR and
GPT partitioning. AFAIK, with the more recent -CURRENT and -STABLE it
is also possible to boot off a root filesystem located on raidz/raidz2
pools. But what about booting off pools consisting of multiple striped
mirror or raidz vdevs? Like this:

Assume each disk looks like a half of a traditional ZFS mirror root
configuration using GPT

1: freebsd-boot
2: freebsd-swap
3: freebsd-zfs

|disk1+disk2| + |disk3+disk4| + |disk5+disk6|

My logic tells me that while booting off any of the 6 disks, boot0 and
boot1 stage should obviously work fine, but what about the boot2
stage? Can it properly handle booting off a root filesystem thats
striped across 3 mirror vdevs or is booting off a single mirror vdev
the best that one can do right now?


- Sincerely,
Dan Naumov

On Sat, Feb 13, 2010 at 12:04 PM, Dan Naumov <dan.naumov@gmail.com> wrote:
> Hello
>
> I have succesfully tested and used a "full ZFS install" of
FreeBSD 8.0
> on both single disk and mirror disk configurations using both MBR and
> GPT partitioning. AFAIK, with the more recent -CURRENT and -STABLE it
> is also possible to boot off a root filesystem located on raidz/raidz2
> pools. But what about booting off pools consisting of multiple striped
> mirror or raidz vdevs? Like this:
>
> Assume each disk looks like a half of a traditional ZFS mirror root
> configuration using GPT
>
> 1: freebsd-boot
> 2: freebsd-swap
> 3: freebsd-zfs
>
> |disk1+disk2| + |disk3+disk4| + |disk5+disk6|
>
> My logic tells me that while booting off any of the 6 disks, boot0 and
> boot1 stage should obviously work fine, but what about the boot2
> stage? Can it properly handle booting off a root filesystem thats
> striped across 3 mirror vdevs or is booting off a single mirror vdev
> the best that one can do right now?
>
>
I don't know, but I plan to test that scenario in a few days.

Matt

> I don't know, but I plan to test that scenario in a few days.
>
> Matt
Please share the results when you're done, I am really curious :)

> It *should* work... I made changes a while back that allow for multiple
> vdevs to attach to the root.  In this case you should have 3 mirror
> vdevs attached to the root, so as long as the BIOS can enumerate all of
> the drives, we should find all of the vdevs and build the tree
> correctly.  It should be simple enough to test in qemu, except that the
> BIOS in qemu is a little broken and might not id all of the drives.
>
> robert.
If booting of a stripe of 3 mirrors should work assuming no BIOS bugs,
can you explain why is booting off simple stripes (of any number of
disks) currently unsupported? I haven't tested that myself, but
everywhere I look seems to indicate that booting off a simple stripe
doesn't work or is that "everywhere" also out of date after your
changes? :)


- Sincerely,
Dan Naumov