Can GEOM be used to intercept the I/o calls to an existing mounted device?

[Tapan Chaudhari]

Thanks Mark, but unmounting will anyways work. But the problem here will be,
a new device "/dev/mirror/gm0" and "/dev/mirror/gm0s1" will be created after
:-

#gmirror label -v -n -b round-robin gm0 /dev/md0
#gmirror load
#gmirror configure -a gm0

and "/dev/md0s1" gets deleted.
Now I will have to mount the new device "/dev/mirror/gm0s1" to get my
original contents.
Can't I do it on the fly while the device is already mounted? I just want
that all the calls going to the device "/dev/md0s1" first come to me and
then it goes to the original device. In short intercepting the i/o calls. Is
there some way I can achieve this?

Thanks,

On Thu, Jul 10, 2008 at 6:39 PM, Mark Gladman <mark at legios.org> wrote:

> On Thursday 10 July 2008 22:41:25 Tapan Chaudhari wrote:
> > Hi All,
> >     I am a newbie to FreeBSD and GEOM. I wanted to intercept the i/o
> calls
> > going to a particular mounted device by writing some driver.
> > I came across  geom and thought it might work. I tried to create a new
> > device, mount it and then tried using gmirror to mirror the device :-
> >
> > #dd if=/dev/zero of=file1 bs=1M count=10
> > #dd if=/dev/zero of=file2 bs=1M count=10
> > #mdconfig -f file1
> > #mdconfig -f file2
> > #fdisk -B /dev/mirror/md0
> > #newfs /dev/mirror/md0s1
> > #mkdir /mnt/mirror0
> > #mount /dev/mirror/md0s1 /mnt/mirror0/
> >
> > #gmirror label -v -n -b round-robin gm0 /dev/md0
> > gmirror: Can't store metadata on /dev/md0: Operation not permitted.
> >
> > It gave me this error "gmirror: Can't store metadata on /dev/md0:
> Operation
> > not permitted."
> >
> > Am I doing something wrong? Or GEOM does not permit me intercept the i/o
> > calls? If not geom, is there any other mechanism by which I can achieve
> it?
> >
> >
> > Thanks,
> > --Tapan.
> > _______________________________________________
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>
> Hey,
>
> You're attempting to set up gmirror on a mounted device.
> Unmount /dev/mirror/md0s1 then retry the gmirror command and it should
> work.
>
> Cheers!
> Mark
>