removable devices auto umounting

[Da Rock]

On Thu, 2008-03-20 at 08:43 +0100, Roland Smith wrote:
> On Thu, Mar 20, 2008 at 11:55:32AM +1000, Da Rock wrote:
> > I'm just looking into the removable device issue for freebsd. I can see
> > its easy enough to auto mount a removable device (although I could use
> > some help getting sd/xd devices working with my card reader), but the
> > removal seems to come unstuck.
> > 
> > I have some barely literates on my systems, so I do need to work this
> > out. Is it possible to use a forced umount to do this? What are the
> > options here?
> 
> In short, no. Removal of a USB device would be forwarded to devd(8). But
> since the device is no longer there at that moment, you cannot unmount
> it anymore. You might get a nice kernel panic for your efforts, though. ;-)
> 
> The FreeBSD disk subsystem was simple not written with removable devices
> in mind, because they didn't exist back then. Until that code is fixed
> (which is hard) you _have_ to unmount before you pull the device out.
> 
> One (not bullet-proof) workaround might be to use the automounter
> [amd(8)], and have it unmount very quickly after they stop being
> active. This requires setting both the 'cache_duration' and
> 'dismount_interval' options in amd.conf(5) to very low values.
>  
> Roland

So by active you mean device access? Or device physical connection? If
its simply access, than that would be perfect- user enters the mount
point, reads or writes a file, amd times out after X secs and dismounts
the device. Physical could be a bit harder...

Also, what docs/how-to's would you suggest for AMD? I looked at the man
and some freebsd doc pages, but another viewpoint would help.
Specifically some more docs on the settings you mention.

Bullet-proof is not exactly necessary- nice, but not critical.
Suggestions for bullet-proof are very welcome though. What is the worst
that can happen if dismounting is not entirely successful? Keeping in
mind that this is mostly a desktop system.