kern/88518: cannot mount root rw at boot

Brad Huntting huntting at
Sat Nov 5 00:30:16 PST 2005

>Number:         88518
>Category:       kern
>Synopsis:       cannot mount root rw at boot
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Nov 05 08:30:14 GMT 2005
>Originator:     Brad Huntting
>Release:        6.0-RELEASE
              The machine in question has problems booting the 6.0-RELEASE 'boot-only' CD.  But it can boot diskless over the network.

The procedure listed below works when installing 5.4-RELEASE, but not on 6.0-RELEASE.  The problem is that the kernel is mounting the root partition readonly even when the root partition is an 'md' pseudo-device.

A hack in 5.4's kern/vfs_mount.c causes mountroot to use RW when mounting an md device.  This appears to be missing in 6.0.

But what's more disturbing, is that the kernel flag (was it -w ?) for 'mount the root partition readwrite' appears to be missing in 5.4 and probably 6.0.  This was a very usefull flag, did it change?  Why was it removed?
        Copy the 'boot-only' cd to a tftp- and nfs-exported directory and setup dhcp.conf accordingly.  For example:

  filename "/scratch/diskless/FreeBSD/6.0R/boot/pxeboot";
  option root-path "";

Assuming you have permissions set correctly in /etc/exports, and tftp enabled from /etc/inetd.conf, a machine which net-boot's should get thrown into the FreeBSD install procedure.

              Either restore the -w flag (or make a kernel environment switch 'boot_readwrite'), or reinstitute the hack in vfs_mount.c that causes 'md' root filesystems to be mounted readwrite initially.

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