How can I make root filesystem read-only?

[Hiroki Horiuchi <hiroki@air.ne.jp>]

Hello.

My /etc/fstab is like below.

/dev/sda1 /     ext2 errors=remount-ro 0 1
/dev/sdb1 none  swap sw                0 0
/dev/sdc1 /tmp  ext2 defaults          0 2
/dev/sdd1 /var  ext2 defaults          0 3
/dev/sde1 /home ext2 defaults          0 4

I am trying to make the root filesystem including /usr subdirectory
read-only. But, if I set the mount option of / to ro, system cannot boot.
Making only /usr read-only is not enought for me.
Cannot root filesystem be read-only?

Thanks in advance.

///Hiroki Horiuchi