On Sun, 31 Dec 2006, Tim Kientzle wrote:
Robert Watson wrote:
... It used to be that only certain file systems could be used as a root
file system, because only they knew how to bypass the lookup procedure to
find their device node, short-circuiting to the in-kernel device list.
So why are the MD_ROOT and NFS_ROOT options still around? It sounds like
there must still be something special about root-capable file systems.
NFS_ROOT has to do with extracting the root mount configuration information
from the interface list, loader environment, prior to the file system mount
process so that NFS knows where to go to find a file system, configure
networking, etc.
MD_ROOT has to do with setting up the md device to mount the root file system
from in memory without having mdconfig run. A glance at the md(4) source code
suggests that it also tweaks the default root device to be "ufs:/dev/md0".
This is not actually a file system option so much as an option in the behavior
of the md(4) device driver.
At least, this is my understanding from a very casual glance at the source.
In both cases, we now use the single vfs_mount VFSOP.
Robert N M Watson
Computer Laboratory
University of Cambridge
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