Re: can "rbd unmap" detect if device is mounted?

[Josh Durgin <josh.durgin@inktank.com>]

On 07/16/2012 12:59 PM, Travis Rhoden wrote:
> Hi folks,
>
> I've made this mistake a couple of times now (completely my fault,
> when will I learn?), and am wondering if a bit of protection can be
> put in place against user errors.

Yeah, we've been working on advisory locking. The first step is
just adding an option to lock via the rbd command line tool, so you
could script lock/map and unmap/unlock.

This is described a little more in this thread:

http://comments.gmane.org/gmane.comp.file-systems.ceph.devel/7094

> I mapped a device "rbd map", then formatted and and mounted the device
> ("mkfs.extf /dev/rbd0..., mount /dev/rbd0...").  Then sometime later,
> I want to remove the RBD device.  Stupidly, I do the "rbd unmap"
> command before I unmount the device.  The kernel doesn't really care
> for this.  Or more accurately, I can't remap that same RBD because I
> run into:
>
> kernel: [2248653.941688] sysfs: cannot create duplicate filename
> '/devices/virtual/block/rbd0'
> ....
> kernel: [2248653.941833] kobject_add_internal failed for rbd0 with
> -EEXIST, don't try to register things with the same name in the same
> directory.
>
> At this point, the "rbd map" command hangs indefinitely (producing the
> logs from above).  Ctrl-C does exit out, though.  But if I try to fix
> my mistake by doing the unmount now, I get the error:
>
> umount: device is busy.
>
> So really I get stuck.  I can't unmount without the device, and I
> can't remap the device to the old block device.  I have to reboot to
> clean up and move on.

A similar issue was fixed in 3.4 (see 
http://tracker.newdream.net/issues/1907).
What kernel are you using? 3.2 had a nasty possibility of preventing
further operations if mapping hung while trying to connect to the
monitors.

> I imagine other bad things can happen with the block device goes away
> out from under the mount point.  Any way the "rbd unmap" command can
> detect when the device is in use or mounted and inform the user?

Before actually unmapping the device, rbd unmap could check if it was
present in mtab. If it's being used as a raw block device and not
mounted or you created and used your own device node this wouldn't
help, but it would be better than nothing.

Josh