Hello list,

I have a particularly odd problem, where I can't figure out what's going on, or 
whether I'm possibly doing something
stupid...

Short summary:
- Supermicro X10DRI-T motherboard, using UEFI boot, 128G RAM, 2 CPUs
- 2 Intel DC P3700 NVME cards, to be used as mirrored zfs root and mirrored log 
devices for a data pool
- FreeBSD-11.1 release installs fine off memstick, and the built system boots 
correctly, but this system uses the entire
  disks (so I need to shrink the system to make room for log partitions)
- so I then rebooted into usb live system, and did a gpart backup of the nvme 
drives (see later). zfs snapshot -r, zfs
  send -R > backup, gpart delete last index and recreate with shorter size on 
both drives, recreate zroot pool with
  correct ashift, restore with zfs receive from backup, set bootfs, reboot
- the rebooting system bootloader finds the zroot pool correctly, and proceeds 
to load the kernel. However, when it's
  supposed to mount the root filesystem, I get:
    Trying to mount root from zfs:zroot/ROOT/default []...
    Mounting from zfs:zroot/ROOT/default failed with error 6.
- when I list the available boot devices, all partitions of the nvme disks are 
listed
- I can import this pool without any issues with the usb live system, there are 
no errors on import.
- I then redid the whole exercise, but restored the previously backed up 
partition tables (full size zfs
  partition), did exactly the same steps as described above to restore the 
previous zfs filesystem, rebooted, and the
  system started up normally.

So, my impression is: the kernel is doing something odd when trying to import 
the zroot pool, if that pool isn't 
using the whole physical disk space. Is there some way to enable debugging for 
the "mount a root zfs pool" process of
the kernel, so see where this is failing?

Here's a more detailled history of my commands to reduce the pool size:

{I'm on a data pool directory here, where I can store stuff. I've booted from 
usb stick into live system}
zpool import -Nf zroot
zfs snapshot -r zroot@backup
zfs send -R zroot@backup > zroot.zfs
zpool destroy zroot
gpart delete -i 3 nvd0
gpart delete -i 3 nvd1
gpart add -a 4k -s 50G -t freebsd-zfs -l zfs0 nvd0
gpart add -a 4k -s 50G -t freebsd-zfs -l zfs1 nvd1
sysctl vfs.zfs.min_auto_ashift=12
zpool create -f -o altroot=/mnt -o cachefile=/var/tmp/zpool.cache zroot mirror 
/dev/gpt/zfs0 /dev/gpt/zfs1
zfs receive -Fud zroot < zroot.zfs
zpool set bootfs=zroot/ROOT/default zroot
reboot

Also, here's a diff between full and partial gpart backup:

--- nvd0.gpart  2017-10-20 14:04:26.583846000 +0200
+++ nvd0.gpart.new      2017-10-20 15:39:20.184445000 +0200
@@ -1,4 +1,4 @@
 GPT 152
 1            efi        40    409600 efiboot0 
 2   freebsd-swap    411648  33554432 swap0 
-3    freebsd-zfs  33966080 747456512 zfs0 
+3    freebsd-zfs  33966080 104857600 zfs0 


I'm really at a loss to see the reason why this is failing. I can provide 
detailled dmesg output of the booting system,
and screenshots of the kvm-efi-console of the failing boot. 

Thanks for any help:)

Cheers,
Markus
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