Re: Mirror a file system on the fly

[Ram Pai <linuxram@us.ibm.com>]

On Thu, 2005-08-18 at 13:27, Dave Schwartz wrote:
> Hi Ram,
> Thanks for the inputs. I was going over the man pages describing the
> clone system call and its option of CLONE_NEWNS. Could understand the
> description only in parts.
> 
> The man page suggests that this flag when set, the cloned child is
> started in a new name space, initialized with a copy of the parent.
> Now does that mean, a program like a shell when cloned with
> CLONE_NEWNS set, will have a copy of file hierarchy of the underlying
> parent process?

Yes the child process will see an exact copy of all the mounts of
various filesystems as that of the parent. However if you mount/unmount
any filesystems in the child, the same will not be mounted/unmounted in
the parent and vice-versa.  Each has its individual view of the
the filesystem heirarchy.

Try the following program that clones off a child process with a mirror
namespace and gives you a bash prompt. Try mounting and unmounting
in this bash prompt and see if the same is visible in a totally
different window.


#include  <stdio.h>
#include  <signal.h>
#include  <sched.h>

char somemem[4096];

int myfunc(){
        system("bash");
}

int
main(int argc, char *argv[])
{
        if(clone(myfunc, somemem, CLONE_NEWNS|SIGCHLD, NULL)) {
                wait(NULL);
        } else {
                printf("clone failed\n");
        }
        printf("exit\n");
}


Hope this helps,
RP




> 
> Gracias,
> decebel
> 
>   
> 
> On 8/19/05, Ram Pai <linuxram@us.ibm.com> wrote:
> > On Thu, 2005-08-18 at 12:40, Dave Schwartz wrote:
> > > Hi list,
> > >
> > > Not too sure if this is the right forum to ask this question but since
> > > my requirement is around linux filesystems, I shall take this liberty
> > > to post my question.
> > >
> > > My requirement is to develop a kernel/user space module to add an
> > > extension to the shell program environment such that this shell forks
> > > a mirror look-alike filesystem of the underlying OS to the programs
> > > run in that particular shell.
> > 
> > u seem to be talking about namespaces, if I get you right.
> > 
> > there is a flag CLONE_NEWNS to the system call 'clone' which does what
> > u r talking about.
> > 
> > RP
> > 
> > 
> > 
> > 
> > >
> > >
> > > Was trying to look thru the FAQ and a few list archives to look for
> > > ideas around my requirement. The archives were overwhelming.
> > >
> > >
> > > Any ideas/pointers will be a great help,
> > > Gracias,
> > > decebel
> > > -
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> >
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