umounting md backed jail filesystems - busy

[joris dedieu]

2011/8/10 Ian Downes <ian at ndwns.net>:
> Hi everyone,
>
> I'm trying to cleanup after shutting down some jails but I'm getting device
> busy errors when trying to umount some of the filesystems.
>
> More specifically, I've got an ephemeral zfs filesystem that serves as the root
> of the jail. On '/etc/rc.d/jail stop' the jail stops cleanly but when I try to
> destroy the zfs filesystem the initial umount fails, claiming it's busy. This
> happens everytime.
>
> I can't for the life of me work out who's tying it up. I've tried fstat, lsof
> and fuser but nothing is reported as active! No processes, no active files.

What gives top -S -n 10000  |grep -i zfs  while system is not
unmountable (could you see some processes in zfs state) ?
What gives jls during this time (does it report anythink still alive) ?

Regards
Joris
>
> Details:
>
> $ uname -a
> FreeBSD XXX.com 8.2-RELEASE FreeBSD 8.2-RELEASE #0: Tue May 31 19:05:32 UTC 2011     root at XXX.com:/usr/obj/usr/src/sys/XENHVM  amd64
>
> 'data' is a md backed zpool
>
> $ mount | grep
> data/path/to/jail/root on /path/to/jail/root (zfs, local)
>
> $ fstat -f /path/to/jail/root
> USER     CMD          PID   FD MOUNT      INUM MODE         SZ|DV R/W
>
> $ unmount /path/to/jail/root
> cannot unmount '/path/to/jail/root': Device busy
>
> Some time later, measured in minutes, something frees up and I can
> umount/destroy the filesystem ok.
>
> Can anyone offer some suggestions on what it could be or other ways to
> determine what's going on?
>
> Thanks,
>
> Ian
>
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