I'm not sure about union fs, never had a real use for it yet.... I have a 
similair setup as you. I have about 4 jails running so far (~8 more to go). 
I've written a fairly large bash scipt to build the jails + configure them 
automagically too. I've got a 120GB drive in that system, so for me I don't 
really have a problem with space. What I use nullfs for is sharing data 
between jails, for example:

/secure/files/mail <- mail spools

would be shared using: (/etc/crontab entries)

none /secure/files/mail /secure/internal/smtp/postfix/server/var/spool/mail 
rw,noexec,nosuid,nodev 0 0
none /secure/files/mail /secure/internal/imap/courier/server/var/spool/mail 
rw,noexec,nosuid,nodev 0 0

haven't really got these up and running yet (ie. I don't really know if 
postfix + courier both work inside a jail). ATM I'm stuck on my file server 
jail, which may never work. I do have djbdns cache+server running. Anyway, 
nullfs is great for "remounting" parts of the file system. I would not use 
that to remount parts of the file system that have executables on them in rw 
mode, only ro. You could mount the base / fs using nullfs onto the jail / 
filesystem but this would be tricky because you would have to deal with 
things like syslogd base + syslogd(s) jail both writing to /var/log/. You 
could mount each directory using nullfs (/bin /sbin, etc) onto the jails and 
I think this would work. I would note the warnings in the man pages about 
this stuff being experimental tho. You could save space by using the same 
executables and libs while having seperate /var/ /etc/ etc dirs in the jail 
but it would be a bit of work to figure out what dirs to mount_nullfs and 
what dirs to have as real dirs. Oh, btw, I know about as much about fs's as 
you do too, so your milage may vary...


On August 3, 2003 04:09 pm, Lewis Thompson wrote:
> Hi,
>   I currently have a bunch of jails running on my FreeBSD box.  I've
> done this by making installworld a number of times, each time with a
> different DESTDIR (say /jail1, /jail2, /jail3).  Clearly this is using a
> significant amount of space on the machine.
>   I've been reading about unionfs and nullfs (well, more skim reading
> really; I'm not FS guru, which is why I'm asking here) and one of these
> sounds like it could be the idea solution.  At first glance I'd say that
> unionfs would be the way to go.
>   My question about unionfs:  if I use this as a base dir for all of my
> jails and decide to ``upgrade'' the base system will it actually work?
> I mean, when I start installing stuff through the ports does it ever
> modify the base system in any way?  If it does, then surely a base
> system upgrade will appear to leave the old ports-created files (because
> the upper layer changes override the lower unionfs fs).
>   Secondly, I don't really understand nullfs.  Would this be a
> ``better'' solution than using unionfs?  Maybe it isn't even a solution,
> but if it is, a pointer to some useful articles would be great (aside
> man mount_nullfs; I've read that but don't fully understand).
>   Thanks very much!
> -lewiz.

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