> 
> I have an existing FreeBSD v4.7 installation and want to delete a small
> filesystem that I had designated for email (/var/mail), and want to freeup
> that unused space to the '/usr/home' filesystem.  If I use DISKLABEL to
> delete '/var/mail', and then re-write '/usr/home' to reflect the added
> space (writing the changes to the SAME '/usr/home' partition), will that
> erase the '/usr/home' filesystem and all of its user-data contents
> from the disk itself???

If I understand what you are saying, you have two (ingoring others
that are not involved such as root) file systems /var/mail and /usr/home
and you want to combine the space from /var/mail in to /usr/home.

By filesystems I do not mean just directories within other filesystems.
They are separately mountable entities.  If they are just directories
within their respective filesystems eg you have /var and /usr filesystems
and mail is just a directory within /var and home is just a directory
within /usr, then it is a completely different story.

To combine those two filesystems (/var/mail and /usr/home) you need to
repartition the disk using disklabel and then run newfs again.  Running
newfs on the newly combined larger partition will wipe out everything
previously (sort of - it doesn't actually overwrite the data, just the 
superblocks which makes it unreachable by normal means).   

There is something called 'growfs' that can allow a filesystem to grow 
to take in some additional free space without wiping out what was already 
in the filesystem.  I have not yet used growfs, but it might work for you.
I would guess that at the least you would have to use disklabel to remove 
the partition that is used for /var/mail.
Someone else might better comment on growfs.  

The disk space to be added must be addressibly contiguous to the base
filesystem that you are trying to increase.  I believe that is true
for growfs as well as for recreating the filesystem with disklabel and
newfs.   If the two spaces are not contiguous, then you will have to 
redo your who slice partitioning schema and remake all of the partitions
and re-newfs them.

Since I can't say much about growfs, I will comment only on merging
the full re-creation way.  So, presuming the two spaces are contiguous,
NOTE: this is best done in single user mode, but can be done in multi.

make a good backup of the /usr/home filesystem.   Use dump(8)
   dump -0af /dev/nrsa0 /usr/home   (presumes a tape drive.  Change the
                                     device to whereever you dump to)
Check it out with restore.
   mt rewind
   restore -ivf /dev/nrsa0   look around a little, maybe restore a file.

Then unmount the filesystems
   umount /usr/home
   umount /var/mail
Then bring up disklabel
   disklabel -r -e /da0s1         (or whatever the disk slice device is) 
     edit the partition table to combine the two in to one.  
     Example, say disklabel shows something like the following:

   8  partitions:
   #        size   offset   fstype [fsize bsize bps/cpg]
    a:  2514944        0   4.2BSD   2048 16384    89  # (Cyl.    0 - 156*)
    b:  4618240  2514944     swap                     # (Cyl.  156*- 444*)
    c: 35551782        0   unused      0     0        # (Cyl.    0 - 2212*)
    e:  2514944  7133184   4.2BSD   2048 16384    89  # (Cyl.  444*- 600*)
    f: 25903654  9648128   4.2BSD   2048 16384    89  # (Cyl.  600*- 2212*)

     and you want to combine partitions e and f
     Edit it to look like the following 
          (all the stuff after the # is ignored so nuke it)
   8  partitions:
   #        size   offset   fstype [fsize bsize bps/cpg]
    a:  2514944        0   4.2BSD   2048 16384    89  # (Cyl.    0 - 156*)
    b:  4618240  2514944     swap                     # (Cyl.  156*- 444*)
    c: 35551782        0   unused      0     0        # (Cyl.    0 - 2212*)
    e: 28418598  7133184   4.2BSD   2048 16384        #

Or better yet, if you are using a recent FreeBSD version

   8  partitions:
   #        size   offset   fstype [fsize bsize bps/cpg]
    a:  2514944        0   4.2BSD   2048 16384    89  # (Cyl.    0 - 156*)
    b:  4618240        *     swap                     # (Cyl.  156*- 444*)
    c:        *        0   unused      0     0        # (Cyl.    0 - 2212*)
    e:        *        *   4.2BSD   2048 16384        #

The newer disklabel will calculate for you all of the offsets and the 
size of the 'c' partition and the last partition if that is to be all 
of the remaining disk. 

Note in either case you are leaving the other partitons as is (whatever
that may be in your case).

Get out of the editor with a write back (save).  You might want to
check it to make sure it is happy by doing:
   disklabel -r da0s1         which forces it to read the actual label
Then do the newfs
   newfs /dev/rda0s1e         Get the device right or you have big trouble

mount the newly remade filesystem
   mount /usr/home                 (presuming it is already in /etc/fstab
                                    from before)
Restore the backup
   cd /usr/home
   mt rewind
   restore -xf /dev/nrsa0

When it finishes, all should be well.

If the spaces are not contiguous on the disk (as shown by disklabel)
then you will have to back up each partition, adjust the partitioning
for all of the partitions with disklabel.  Newfs each of the newly moved
partitions and then restore the backups.
If it seems complicated, it helps to write down all the commands in
advance.   It is easy to forget a step in the heat of the moment if
you are not used to doing this stuff (even if you are used to it).

If the spaces are contiguous, check out growfs to see if it will
work well.  

If /var/mail and/or /usr/home are not separate filesystems, but just
directories within filesystems, you will have to do a variation on
the whole reshuffle - at least for the filesystems involved.

Good luck,

////jerry
     
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