I just finished a fairly painless upgrade from Zimbra 5.0.18 GA FOSS on Debian 4 (Etch) to Zimbra 6.0.4 GA FOSS on Debian 5 (Lenny). All in all the process took about 3 hours, including a pretty lengthy fsck. Keeping all the notes and steps for posterity's sake.

Step 0: Backup!

Step 1: Install prerequisites

Step 2: Download everything

Step 3: Run the upgrade from Zimbra 5.0.18 to Zimbra 6.0.4 Debian 4

Step 4: Upgrade from Debian 4 (Etch) to Debian 5 (Lenny)

Step 5: Run the Zimbra 6.0.4 Debian 5 installer

Step 6: Apply any optional configuration changes

Step 0: Backup!

I do a daily backup using a script found on the Zimbra wiki that rsync the zimbra directory to a backup directory. For this upgrade, I did a special backup just to get the most current data.

Shutdown all Zimbra processes (as zimbra):

zmcontrol stop 

Start the backup (as root):

rsync -avHK --delete /opt/zimbra/ /opt/backup/zimbra 

Step 1: Install prerequisits

As far as I can tell, Zimbra 6 only requires one additional package called sysstat.

To install the package (as root):

apt-get install systat 

After the package is installed, edit the /etc/default/sysstat file and change the ENABLED variable to true.

Step 2: Download everything

Downloading everything you'll need will make the upgrade fast and smooth. First, you'll need both the 6.0.4 binaries for Debian 4 and Debian 5:

wget http://h.yimg.com/lo/downloads/6.0.4_GA/zcs-6.0.4_GA_2038.DEBIAN4.0.2009...
wget http://h.yimg.com/lo/downloads/6.0.4_GA/zcs-6.0.4_GA_2038.DEBIAN5.200912... 

Might as well also download the files needed to upgrade Debian. First modify your /etc/apt/sources.list and set the target to lenny. My sources.list looks like this:

deb http://debian.lcs.mit.edu/debian/ lenny main contrib non-free
deb-src http://debian.lcs.mit.edu/debian/ lenny main contrib non-free

deb http://security.debian.org/ lenny/updates main contrib
deb-src http://security.debian.org/ lenny/updates main contrib

Then, just use apt-get to download all you need to do a distribution upgrade:

apt-get update
apt-get -d dist-upgrade 

Step 3: Run the upgrade from Zimbra 5.0.18 to Zimbra 6.0.4 Debian 4

Once all the downloads have completed, go ahead and run the install.sh for Zimbra 6.0.4 for Debian 4. Go through the normal Zimbra upgrade procedure. This step took about 45 minutes for me with about 50 mailboxes across 70GB.

Step 4: Upgrade from Debian 4 (Etch) to Debian 5 (Lenny)

This is a fairly straight forward process.

To upgrade Debian from 4 to 5 (as root):

apt-get dist-upgrade 

Since all of the needed packaged have already been downloaded from the previous step, there're just a few configuration menus you need to go through.

Once the upgrade is complete, reboot the box to run the new kernel. For me, this was when a forced fsck was performed on the drive since I haven't rebooted this box for over 8 months. Fsck fixed some minor issues and the system booted up about an hour later.

Step 5: Run the Zimbra 6.0.4 Debian 5 installer

Once you boot into the brand spanking new Debian 5, run the install.sh from the Zimbra 6.0.4 Debian 5 package and proceed with the upgrade. You can skip the table check this time.

Since this is an upgrade to the same version, it only took about 10 minutes for me. Once it's done, make sure all of the processes starts without a problem.

Step 6: Apply any optional configuration changes

Last thing is to reapply any modifications that do not persist across upgrades. For me, it's just the Trend Micro's email reputation service.