Exchange Server 2003 SP2 - storage limits

Boy do people have a problem with this one. What should be tremendously simple has turned out to be quite difficult.

Exchange Server 2003 Standard up to and including SP1. 16GB limit. Hit that, store stops.
SP2 raises this BY DEFAULT to 18GB. BUT you can change this to anything you like that is up to 75 (GB).

START, Run, Regedit. Navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\\

At this point, you cannot change the number as the default is set to 18 in the absence of an entry. To change from the 18GB default, create a new reg_dword and name it “Database Size Limit in GB”. Your default value will be 18 but this can be any number up to 75. Not sure if you can set it to 1, but that would be kind of silly.

IMPORTANT RULE.
Size is everything.

Perform the following maths as a rule of thumb:
X: The size of the volume where your Store lives in GB. If in doubt, Search for *.edb or look in ESM.
Y: Space used up by everything else on that volume EXCEPT the *.edb and *.stm
Z: Store size.

Obviously Y+ZIt would be useful to have another Z left as free space.
It would be preferable to have another 2 times Z left as free space.
It would be even more preferable to have another 3 times Z left as free space.

Why?
Defrags need Z times 1.1 as a minimum.
Email expands.
Disk thrashing / fragmentation will reduce performance significantly as you fill up the disk.

So Z (which is the one you can set in the registry) should really be no more than (X-Y)/2 and preferably no more than (X-Y)/3.

An article to demonstrate this, complete with examples will be posted in my Exchange Server articles when I get a moment to write it up.