umounting md backed jail filesystems - busy

[Ian Downes]

Joris,

thanks for the suggestions but they don't appear to indicate anything unusual.

> > 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) ?

$  jls
JID  IP Address      Hostname                      Path

$ top -S -n 10000  |grep -i zfs
5 root         6  -8    -     0K    88K tx->tx  2   0:01  0.00% zfskern

It's entirely possible that this isn't due to a process related to the jail but
it is zfs tying things up for a while. I'd like to understand what's going on though
as I'd rather not just hack something together to schedule purges at some later
time.

> >
> > 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?