Failed to use read-only NFS as the root filesystem

Failed to use read-only NFS as the root filesystem

[Min Jun,Xi]

Hi All,
there's one problem here.
I tried to set one read-only NFS directory as the root filesystem, but
there're several problems, and lead to one situation that the root
filesystem is remounted using read-write option.
The kernel parameter is here:
+++++++++++
append="root=nfs:192.168.11.107:/install/netboot/rhel6/ppc64/compute/rootimg:ro
STATEMNT= XCAT=192.168.11.107:3001 ip=dhcp"
+++++++++++
#please ignore STATEMNT and XCAT, which are the OPTIONS created by myself.
Before "switch_root" command is running, we can see the root
filesystem is mounted read-only, you can find it here:
++++++++++++
192.168.11.107:/install/netboot/rhel6/ppc64/compute/rootimg on
/sysroot type nfs (ro,nolock,nfsvers=3,addr=192.168.11.107)
++++++++++++
However, after "switch_root" is running, all have been changed like this:
+++++++++++++++++++++++++++++++++++++++++
dracut: Switching root
                Welcome to Red Hat Enterprise Linux Server
                Press 'I' to enter interactive startup.
Starting udev: [  OK  ]
Setting hostname ca4dsls02:  [  OK  ]
Setting up Logical Volume Management:   No volume groups found
[  OK  ]
Checking filesystems
Checking all file systems.
[  OK  ]
Remounting root filesystem in read-write mode:  [  OK  ]
Mounting local filesystems:  [  OK  ]
++++++++++++++++++++++++++++++++++++++++++
Can you find it?  "Remounting root filesystem in read-write mode:  [  OK  ]"
Finally, I found something related with it.
In "/etc/init.d/rc.sysinit", the code looks like this:
+++++
639  Remount the root filesystem read-write.
640 update_boot_stage RCmountfs
641 if remount_needed ; then
642   action $"Remounting root filesystem in read-write mode: " mount
-n -o rem    ount,rw /
643 fi
+++++
It seems the "update_boot_state" function tells us we need to remount
the NFS directory, then it remounts the root filesystem with
read-write option.
Is there any option or flag which can be used to disable it? I think
the developers from redhat should know about it.
Thank you!

Re: Failed to use read-only NFS as the root filesystem

[David Dillow]

On Mon, 2010-07-05 at 00:48 +0800, Min Jun,Xi wrote:
> Hi All,
> there's one problem here.
> I tried to set one read-only NFS directory as the root filesystem, but
> there're several problems, and lead to one situation that the root
> filesystem is remounted using read-write option.

> Is there any option or flag which can be used to disable it? I think
> the developers from redhat should know about it.

Search /etc/rc.d/rc.sysinit for /etc/sysconfig/readonly-root. I don't
know if it is well documented or not. You can also use "readonlyroot" on
the command line I think, but using the file lets you set more options.
Expect more issues related to this, the last time I looked at it, it
took some tweaking to make work well.

Re: Failed to use read-only NFS as the root filesystem

[Steffen Maier]

On 07/04/2010 07:16 PM, David Dillow wrote:
> On Mon, 2010-07-05 at 00:48 +0800, Min Jun,Xi wrote:
>> Hi All,
>> there's one problem here.
>> I tried to set one read-only NFS directory as the root filesystem, but
>> there're several problems, and lead to one situation that the root
>> filesystem is remounted using read-write option.
> 
>> Is there any option or flag which can be used to disable it? I think
>> the developers from redhat should know about it.
> 
> Search /etc/rc.d/rc.sysinit for /etc/sysconfig/readonly-root. I don't
> know if it is well documented or not. You can also use "readonlyroot" on
> the command line I think, but using the file lets you set more options.
> Expect more issues related to this, the last time I looked at it, it
> took some tweaking to make work well.

Even thought it focuses on a specific use case on s390x, the following
document for readonly root-fs on RHEL 5.3 might help you with some more
background information:
http://www.linuxvm.org/Present/misc/ro-root-RH5.pdf

Steffen

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