Aligning MBR for ZFS boot help

[Cody Ritts]

Hello all,

I am really struggling to understand what is going on, if anyone could 
tell me where I am going wrong, I would greatly appreciate it.

I have a new intel atom appliance that will not boot from a GPT 
partition table.  It came with an SSD, so I am trying to align it to 1MB 
for the erase block size.

All of these commands are being run from a 9.1-RELEASE-amd64-memstick


These commands partition the drive, the system boots just fine:
> gpart create -s mbr ada0
> gpart add -t freebsd ada0
> gpart create -s bsd ada0s1
> gpart add -s 52862M -t freebsd-zfs  ada0s1
> gpart add -s 8G  -t freebsd-swap ada0s1
> gpart set -a active -i 1 ada0
> gpart bootcode -b /boot/mbr ada0
> dd if=/boot/zfsboot of=/dev/ada0s1 count=1
> dd if=/boot/zfsboot of=/dev/ada0s1a skip=1 seek=1024

This is the gpart print output of those commands
> =>       63  125045361  ada0  MBR  (59G)
>          63  125045361     1  freebsd  [active]  (59G)
>
> =>        0  125045361  ada0s1  BSD  (59G)
>           0  108261376       1  freebsd-zfs  (51G)
>   108261376   16777216       2  freebsd-swap  (8.0G)
>   125038592       6769          - free -  (3.3M)

Here is my disk info
> root@:/root # diskinfo -v ada0
> ada0
> 	512         	# sectorsize
> 	64023257088 	# mediasize in bytes (59G)
> 	125045424   	# mediasize in sectors
> 	0           	# stripesize
> 	0           	# stripeoffset
> 	124053      	# Cylinders according to firmware.
> 	16          	# Heads according to firmware.
> 	63          	# Sectors according to firmware.

125045361 + 63 = 125045424
So gpart is for sure printing sectors.
freebsd-zfs starts at sector 63

So, I need that freebsd-zfs slice to start at 1MB
1MB = 2048s
2048 - 63 = 1985
so if I add an offset to my slice:
> gpart add -b 1985 -s 52862M -t freebsd-zfs ada0s1

should start me at 2048.
> =>       63  125045361  ada0  MBR  (59G)
>          63  125045361     1  freebsd  [active]  (59G)
> =>        0  125045361  ada0s1  BSD  (59G)
>           0       1985          - free -  (992k)
>        1985  108261376       1  freebsd-zfs  (51G)

BUT, when i boot, I get this:
> zfsboot: No ZFS Pools located, can't boot

I think remember reading that freebsd-zfs had to be the first slice (I 
cannot remember where i read that).  And it apparently does not think an 
offset is funny.

So, that leaves me with trying to adjust my MBR partition, so I start 
over and run:
> gpart add -b 1985 -t freebsd ada0

but that gives me:
> =>       63  125045361  ada0  MBR  (59G)
>          63       1953        - free -  (976k)
>        2016  125043408     1  freebsd  (59G)

HHHMMMMM????  well, 2016 - 1953 = 63  coincidence?  i doubt it, but I 
dont get it.

Poking around on the internet, it looks like gpart is possibly enforcing 
geometry boundaries? so I do the following:

> sysctl kern.geom.part.check_integrity=0
> root@:/root # gpart add -a 1m -t freebsd ada0
> ada0s1 added
> root@:/root # gpart show
> =>       63  125045361  ada0  MBR  (59G)
>          63       2016        - free -  (1M)
>        2079  125042652     1  freebsd  (59G)
>   125044731        693        - free -  (346k)

Obviously still didnt work.


I try a 10MB offset.
10MB = 20480s
20480-63  = 20417s
> gpart add -b 20417 -t freebsd ada0
> =>       63  125045361  ada0  MBR  (59G)
>          63      20412        - free -  (10M)
>       20475  125024949     1  freebsd  (59G)

It is still just a few sectors off.  So what if i let gpart 
automatically align it.

> gpart add -a 1m -t freebsd ada0

> =>       63  125045361  ada0  MBR  (59G)
>          63       2016        - free -  (1M)
>        2079  125042652     1  freebsd  (59G)
>   125044731        693        - free -  (346k)


And 2079 is still != 2048.

I have tried adjusting those numbers one by one, and it just hops around 
the number I am looking for.  I have tried adding partitions in-front of 
it, setting the alignment to 1s, and adjusting the size.  I cannot get 
it to land on 2048.

It does boot with the padding in the MBR table, but I don't think it is 
aligned.  Maybe it is aligned, and I just dont know any better.

I am at a loss.

Any suggestions would be greatly appreciated.

Thanks,

Cody