How does kernel know the root directory?

bala1486 · 03-25-2010, 09:07 PM

Hello All,
I have a question on how the kernel knows the root directory. For example, in grub menu.lst file we specify root=/dev/hda1. This tells the kernel to find mount the root directory from /dev/hda1. But to read /dev/had1 it should first know where is '/' . I couldn't understand how this is being done. Could anybody please explain me?? Thank you....

Thanks,
Bala

Quakeboy02 · 03-25-2010, 09:13 PM

My understanding, which could be wrong, is that when you install grub, it puts a hard link into the MBR (Master Boot Record) to the /boot directory that grub will use. During boot, it queries the menu.lst file, as well as the device.map file, and from them decides where "/" is. Alternatively, if you use disk labeling, then it will search for the disk with the label specified in menu.lst and that will be "/".

kevmcool · 03-26-2010, 04:43 AM

does the line in grub.conf
kernel /vmlinuz................ root=/dev...............
tell the kernel where the / is
root

syg00 · 03-26-2010, 06:00 AM

Have a read of this for some background. Very handy site.
If that leaves unanswered questions, you always have the source to refer to.

jefro · 03-26-2010, 04:17 PM

My thinking would be that by "mounting as root" a partition, no matter where you have the partition, would be root. So that any partition could be mounted as /root

frieza · 03-26-2010, 04:42 PM

isn't it defined by fstab, which is created on install?
eg:

Code:

# /etc/fstab: static file system information.
#
# Use 'blkid -o value -s UUID' to print the universally unique identifier
# for a device; this may be used with UUID= as a more robust way to name
# devices that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    defaults        0       0
# / was on /dev/sda2 during installation
UUID=44169a1f-a1aa-476a-9837-48ccb9853381 /               ext4    errors=remount-ro 0       1
# /boot was on /dev/sda1 during installation
UUID=d43e0044-5a1d-4f17-abc3-c6fd77b20dd7 /boot           ext4    defaults        0       2
# /media/music was on /dev/sdc1 during installation
UUID=11aed5b6-5aa7-4f51-85a6-99017cb89c09 /media/music    ext3    defaults        0       2
# /media/stuff was on /dev/sdb2 during installation
UUID=bbe33dc3-0a1d-43cb-8874-c907b4954129 /media/stuff    ext3    defaults        0       2
# /media/videos was on /dev/sdd1 during installation
UUID=8f115647-e644-4bb4-a250-8cc42fbbe186 /media/videos   ext3    defaults        0       2
# swap was on /dev/sdb1 during installation
UUID=8f14d31e-7c39-40ef-ba63-76ef5ad0426f none            swap    sw              0       0
/dev/scd1       /media/cdrom0   udf,iso9660 user,noauto,exec,utf8 0       0
/dev/fd0        /media/floppy0  auto    rw,user,noauto,exec,utf8 0       0
sshfs#user259713@172.16.0.21:/home/user259713/backend /mnt/modules fuse user,noauto 0 0
sshfs#user874-3985@172.16.0.22:/home/user874-3985/www /mnt/website fuse user,noauto 0 0