[Jafo232]

You can use it to upgrade multiple forums at once if you want to, but I would recommend creating a new source repository and upgrading your forums one at a time just for the reasons you laid out.

Opcode caching is basically this. When a PHP script is executed on your server, the server reads the script, compiles it, and then executes the compiled code (op code). It does this EVERY time the script is called. Of course, there is a lot of overhead compiling the source over and over so caching will only compile it once and then the server will use the compiled code saving you A LOT of memory and CPU.

When you run multiple forums on a server, and you are using opcode caching, each forums file will be cached even though for all intents and purposes, they are the same files. By allowing all the forums to share one set of source files, they are only cached once.

So index.php on forum1 and index.php on forum2 are only cached one time when you use the DLS as opposed to twice. This cuts the memory usage for cached files in half.

If you are not comfortable with your server prowess, you may want to test this on some test installs before you try to do it live.