On 11/16/12 14:07, dweimer wrote:
On 2012-11-15 17:31, Ronald F. Guilmette wrote:
(This stuff would probably be a lot less confiusing if I actually knew
what I was doing, but...)
OK, Warren, I've just done the following steps. The first two I drew
from the manpage examples, and then followed those up with two commands
from your tutorial.
/sbin/gpart create -s GPT ada0 # manpage example is wrong, ad0 ->
ada0
/sbin/gpart bootcode -b /boot/mbr ad0 # manpage wrong again, pmbr
-> mbr
gpart add -t freebsd-boot -l gpboot -b 40 -s 512K ada0
gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0
That last one, done at the suggestion of your tutorial page, has me
completely perplexed, because of what is said, very explicitly, in the
gpart(8) manpage:
bootcode Embed bootstrap code into the partitioning scheme's
metadata on
the geom (using -b bootcode) or write bootstrap code
into a
partition (using -p partcode and -i index).
Please note the use of the word "or".
The man page is telling me to _either_ use the -p option _or else_ use
the -p and -i options together. But you are telling me to use all three
in one go!
Forgive me, but I'm confused. (As you can tell by now, I am often easily
confused. Sorry.)
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I saw this, and well started wondering myself, as I have been using this
while doing work on booting FreeBSD via ZFS (of course using -p
/boot/gptzfsboot), I got the line from a tutorial on booting from ZFS.
Never thought much of it, until now, but I believe I see now why, the
secret is the pmbr, notice the "p". Its the protective mbr, it lets
formatting tools that understand mbr, but not gpart know that there is
something there, the actual boot code is in the partition.
pmbr serves two purposes. It's both the first stage boot code, as a
traditional BIOS always loads the first block of the disk into memory
and runs it to boot the machine regardless of whether you've got an MBR
or GPT disk, and it contains a traditional MBR that shows the entire
disk is occupied by the first DOS partition (slice in BSD terminology)
and that is of type 0xee. The latter means that GPT ignorant utilities
see the disk as fully occupied by a partition of unknown type, which
should mean they won't touch anything.
The pmbr boot code understands the GPT table and runs through the
partition entries looking for one of type freebsd-boot. When it finds
one, it then loads the contents of the partition (or the first 545k if
it's larger) into memory and jumps to the second stage boot loader.
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