Filesystem snapshots

[John <john@commonpeople.uk>]

Advice sought on merging LVM snapshots nightly. I think it's on topic 
for the list and potentially useful for new users reading their way into 
tarsnap - possibly even on the FAQ.

In order to archive data when constant updating takes place, even just 
an email INBOX or a SQL database, the data has to stop changing. The 
frozen unchanging data is the source for the archive and an overspill 
area, called - very misleadingly - a snapshot, can accumulate written 
blocks of fresh data. When the archive is finished, the snapshot can be 
merged back into the original files and deleted.

My instance is a VirtualBox Slackware 14.1 /home partition encrypted 
with LUKS on LVM2.

In order to make space for an additional temporary partition (space for 
the snapshot), I rebooted to init level 3 (a command prompt and no X), 
unmounted /home, reduced the size of the /home filesystem (resize2fs), 
reduced the size of the /home partition (lvreduce) and expanded the 
filesystem back to the partition size (resize2fs).

Nightly crontab can now run tarsnap on a read-only partition while my 
normal activity continues. My root script creates a snapshot of the home 
partition (lvcreate -s). This allows the home partition to appear to be 
mounted read-write while the snapshot partition can be mounted read-only 
and archived (tarsnap --snaptime).

Once the archive is complete, the accumulated new data can be merged 
back into the /home filesystem (lvconvert --merge) which will 
automatically remove the snapshot partition and leave the unallocated 
logical volume space free for the next time.

My puzzle, for which I have no answer and seek advice, is this:

My X system has remained up but logged off throughout this nightly 
crontab root script, and that's kept files open and updating on /home. 
The merge has only been queued by the lvconvert command and will only 
take effect when /home is unmounted and then mounted again. I need to 
automate closing X, unmounting and remounting /home and starting X again 
and I can't think how to do that neatly within the crontab root script. 
I got as far as (killall --user john) which does bring my instance of X 
down very effectively, but it feels too extreme (and it probably still 
leaves /home/john in use). My intention is to be able to log in normally 
and carry on when I arrive at my PC in the morning.

How does anyone on the list handle nightly snapshot archives tidily?