setting root filesystem in kernel

[SteveG1]

Hi,

Is there a way to set the root file system in the kernel? My goal is to boot from one drive and then have it use another drive as root.

Thanks!

 

[SteveG1]

The reason for doing this is so that I can boot off the built in sata controller, and then move over the raid controller. I have not been able to make it boot from raid.

 

[T-Daemon]

You can boot the root file system from a different disk-partition (raid volume) with a kernel loaded from another disk-partition.

When BIOS see boot.config(5).

When UEFI set a loader environment from a configuration file in the ESP. For example, the kernel from disk 1, partition 3 should boot the root file system from disk 2, partition 3, then set in the ESP's /efi/boot/freebsd/loader.env

currdev=disk1p3

rootdev=disk1p3

(In 13.0-RELEASE this doesn't work, presumably due to a bug, but it works in 13.1-RELEASE.)

 

[Alain De Vos]

There are 3 variables in loader.conf
E.g.

loaddev="disk2p9:"
currdev="zfs:ZT/ROOT/default:"
vfs.root.mountfrom="zfs:ZT/ROOT/default"

 

[T-Daemon]

Setting currdev= in /boot/loader.conf is a better solution than boot.config(5) or loader.env in ESP, independent if the firmware is BIOS or UEFI.

To quote from loader(8):

     currdev   Selects the default device to loader the kernel from.  The
               syntax is:
                     loader_device:
               or
                     zfs:dataset:
               Examples:
                     disk0p2:
                     zfs:zroot/ROOT/default:

(loader_device -> UFS2)

 

[T-Daemon]

To clarify, there are two loader variables, currdev and rootdev, which can be used to set the root file system (see loader(8) for details).

The currdev variable selects the default device/partition from which the kernel should be loaded and the same root file system be mounted.

Example: The root file system where the variable is set is on disk 1 and the kernel and root file system which should be booted from and mounted is on disk 2, partition 2: currdev=disk1p2: (disk counting starts at 0), ZFS currdev=zroot1/ROOT/default: (zroot1 pool being here the file system on disk 2).


The rootdev variable specifies only the root file system to be mounted, not the kernel to be loaded.

Example: The kernel to boot from and the variable set is on disk 1 and the root file system which should be mounted is on disk 2, partition 2: rootdev=disk1p2:.

rootdev works well with UFS2, on ZFS it doesn't ( tested on 13.1-RELEASE). Neither the kernel nor the root file system specified is hand over as variable to the loader. Instead set vfs.root.mountfrom="zfs:zroot1/ROOT/default".

With the rootdev or vfs.root.mountfrom variable set, the kernel and and the mounted root file system must have the same version..

 

[avner]

I am using UFS2 and had no success in setting currdev in /EFI/freebsd/loader.env but did have success setting rootdev even though kernel and root file system were on a different HDD than the ESP. I have not tried yet to set either of these variables in /boot/loader.conf. Maybe they would work there.