Other Root on hammer2

Annalesa · 2026-10-09T17:37:48+0100

It's in a bhyve guest:
I installed hammer2 by pkg install hammer2.
Then

 kldload hammer2

newfs_hammer2 /dev/nda1

mount -t hammer -o hflags=0 /dev/nda1 /mnt

and installed FreeBSD-15.1-RELEASE on /mnt.
I use a separate /boot because the efi loader dose not support hammer2.
It's my /boot/loader.conf

 root@anna:~ # cat /mnt/boot/loader.conf

zfs_load="YES"

hammer2_load="YES"

vfs.root.mountfrom="hammer2:/dev/nda1"

vfs.root.mountfrom.options="hflags=0"

But other PFS like /dev/nda1@AAA cannot be mounted in the mountroot phase in this way.
And I'm trying to find a workable method

 root@anna:~ # hammer2 stat /

PATH             ncp  data-use inode-use comp               check        quota

/                  0  469.22MB     10598 lz4:default        xxhash64

root@anna:~ # mount

/dev/nda1 on / (hammer2, local)

devfs on /dev (devfs)

zroot/var/log on /var/log (zfs, local, noatime, noexec, nosuid, nfsv4acls)

zroot/ROOT/test on /mnt (zfs, local, noatime, nfsv4acls)

covacat · 2026-10-09T19:50:46+0100

i tried it but it insta panics on write
newfs, mount, pfs-create works but a write will cause a panic
i tried on 14.5 on arm64. im not sure what causes the problem version or arch

T-Daemon · 2026-10-09T20:07:09+0100

Annalesa said:
But other PFS like /dev/nda1@AAA cannot be mounted in the mountroot phase in this way.
And I'm trying to find a workable method

Why not mount PFSs via /etc/fstab as described in the DragonFly BSD handbook, chapter Manual Installation with an Extended HAMMER2 Disk Layout? Scroll down to "Describe all mounts in fstab:".

covacat · 2026-10-09T21:56:36+0100

if i disable lz4 with hammer2(8) works a bit better like it not panics but hangs on umount
seems broken on arm because upgrading to 15.1 did not change anything to the better

Annalesa · 2026-10-10T12:29:01+0100

T-Daemon said:
Why not mount PFSs via /etc/fstab as described in the DragonFly BSD handbook, chapter Manual Installation with an Extended HAMMER2 Disk Layout? Scroll down to "Describe all mounts in fstab:".

It's the mountroot stage, /etc is on hammer2, so if you have not mounted hammer2 on /, there is no /etc

covacat · 2026-10-10T12:36:58+0100

ok
freebsd kernel will check if the device exists before mounting anything but p9, zfs or nfs so a device spec like /dev/disk@pfs wont work because obviously disk@pfs does not have a node
here is the hack you can do
vfs.root.mountfrom="hammer2:/dev/nda1"
vfs.root.mountfrom.options="hflags=0,from=/dev/nda1@mypfs"

covacat · 2026-10-10T12:39:07+0100

Annalesa · 2026-10-10T13:00:27+0100

covacat said:
ok
freebsd kernel will check if the device exists before mounting anything but p9, zfs or nfs so a device spec like /dev/disk@pfs wont work because obviously disk@pfs does not have a node
here is the hack you can do
vfs.root.mountfrom="hammer2:/dev/nda1"
vfs.root.mountfrom.options="hflags=0,from=/dev/nda1@mypfs"

Thanks, so that’s possible too. Actually, I modified sys/kern/vfs_mountroot.c and added strcmp(fs, "hammer2") == 0 || after zfs, nfs and p9fs that don’t need to be checked, and recompiled the kernel.

cracauer@ · 2026-10-10T13:48:02+0100

If you get a panic it would be great to post the backtrace.

covacat · 2026-10-10T15:35:51+0100

its about 5000 calls to something like lz4xxx64klmitedoutput. after i set lz4 to off it works a bit better. the backtrace is not really useful. so the stack seems borked because the function itself is not recursive

Alain De Vos · 2026-10-10T15:40:33+0100

hihi when you pass a message