If you point more than one worker process at a single DNN install then you will also be running multiple instances of the scheduler and multiple application caches.

The multiple application caches holding the same "global" objects is why it takes a lot more memory.

Running redundant scheduler tasks at the same time is also bad for performance if it doesn't lock up completely.

Steve Taylor wrote Are you referring to WS2003 Task Scheduler or some other scheduler within DNN? - sorry for the confusion, I am still learning about DNN internals.

The DNN scheduler.

Steve Taylor wrote With regard to memory overhead, doesn't it amount to much the same amount of memory whether you implement two sites (say) using a single DNN installation with 2 portals each running in a different application pool (2 processes) or two separate DNN installations, one for each site (2 processes)?

Yep, just about.

Steve Taylor wrote Taking on board your ealier point about the reasons for not using separate portals within a single DNN installation to implement separate sites, I did not follow what you meant when you said:  They will not be sand-boxed at the physical file system so access to the file system through FTP can not be granted.

Since they share the same physical file locations, granting FTP access to the install would allow any one to alter everyon'es setup.  You can grant FTP access to just the portal levels but, in reality, managing a DNN site/portal doesn't need FTP except for upgrading.

Steve Taylor wrote And the points made by Darrin McQuay and Chris Hammond regarding the need for a separate IP address for each site / portal if using SSL would presumably also apply to the case of two separate DNN installations, if each were running one of the two sites each with its own domain name?

The separate IP for SSL is an IIS requirement, not DNN.  Though a wildcard SSL would work, it wouldn't be as easily trusted.

Steve Taylor wrote My worries about disk-space are unfounded, I think, since I just looked at the size of my DNN installation on my VPS and, from the 4.8.2 install (without source) and with no extra modules installed and a single portal with no extra pages or skins it comes to about 23MB, so even after adding pages, modules, skins etc, I could probably fit a couple of hundred of these on my VPS. Clearly, memory and availability of IP addresses are going to be much more significant limiting factors. My current provider only allows one IP address per VPS, so it looks like I'm going to be looking for another provider soon - any suggestions welcome.

Physical RAM is king, and unfortunately usually costs a premium in a virtual server.  SQL space can be important as well.

But you're definitely on the right track.

Jeff

Interesting new certificates - I was not aware of them but will certainly bear them in mind.

Thanks to all who contributed to this thread - I found it interesting and informative. I has also changed my way of thinking about the appropriate use of portals in DNN.

Take care,

Steve Taylor