ZFS boot pool selection

[Aristedes Maniatis]

I'd like to better understand the boot process as it applies to a ZFS on root approach in FreeBSD 9 or 10 using GPT. What I understand so far:

A. BIOS is able to execute some code placed in a special partition on a GPT formatted disk. This code is 40kB of hand crafted code and installed using:

gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0

The partition itself must be created as

gpart add -s 222 -a 4k -t freebsd-boot -l boot0 ada0


B. For older BIOS systems without knowledge of GPT, the pmbr is installed in some other special location on the disk, outside any partitions. This is 512 bytes of code and does nothing other than pretend to be MBR to tell the old BIOS (or Windows?) to not mess with this disk.

C. Once gptzfsboot is executing on the CPU, it is able to mount a ZFS pool in read only mode. Enough to read the kernel and get a full ZFS implementation in place.



Questions

1. If I have two zpools on this machine, how does gptzfsboot know which one to boot the kernel from? Does it just start by iterating through all zfs partitions until it finds the zpool metadata cache which give it enough information to mount some zpool? In other words, does it pick a random pool from what it can access?

2. How does it know where to find the kernel once it mounts the ZFS pool, or is the /boot/kernel location hardcoded into the gptzfsboot code?

3. Once the kernel is booted, then it can read /boot/loader.conf. In there we can see vfs.root.mountfrom="zfs:tank" but isn't this a bit late? We've already mounted this pool and loaded the kernel from it. Can we omit vfs.root.mountfrom entirely?


I understand grub a little better, since in that case you configure, then install a new configured grub artifact into the right places. But with the FreeBSD boot process there seems to be no configuration of the first stage boot loader.


Thanks very much for helping me to understand this boot process in more detail


Cheers
Ari


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Aristedes Maniatis
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