I'm thinking about getting a vps to experiment some stuff. I have three websites that I want to migrate, completely unrelated.
From a resource point of view I assume a multisite installation is cheaper than three different installation, am I wrong? is a multisite one good if the site are totally different in plugins, admins,themes and so on?
also, is a multisite (of 3 identical site) more expensive han a singlesite 3 times bigger? why is that?
thank you
I am not aware of any significant performance differences between using a multi-site and several distinct instances of WordPress. Nor have I heard of performance being an incentive or disincentive to using a Multi-Site.
I cannot give you a definitive answer as to whether or not there is a performance difference, but I'd be willing to bet it is negligible.
The real reason you use a multi-site is to make management easier. It's also great if you need to share users across multiple sites; a multi-site has a single user table, but separate tables for posts, options, categories, etc.
Regarding your other question:
is a multisite (of 3 identical site) more expensive han a singlesite 3 times bigger?
I'll assume that by bigger you mean more posts.
In terms of storage there is not going to be a significant difference. In terms of performance there can be.
Multi-sites have separate tables for posts for each individual site. When you're viewing an individual site within a multi-site, queries are only being run on one of those tables. With respect to performance, this question boils down to:
Is the performance of a site A better than the performance of a site B if site B has three times as many posts?
The answer to that seems obvious. However, it is important to note that database performance is not linear with respect to database size. Queries on a site A are not going to be three times faster than site B (typically).
The comparison doesn't really say anything about multi-sites. There is no way to break the larger site up into several sites in a multi-site and there is no way to combine the multi-site sites into a large single site.
Update:
from a security point of view I assume different instances of wordpress are safer? if one of my site admins gets his account stolen and installs malicious code,it should have an harder time infecting the other installations,right?
Assuming the separate instances are installed on the same server, it may be somewhat harder to attack all three sites compared with a single multi-site. However, once someone is able to execute their own code on your server, it is trivial to expand an attack to other sites on the same server.
While it may be a bit harder for an attacker to compromise three separate instances of WordPress, it is also harder to secure those separate instances. With a multi-site you can easily deploy security measures across all of your sites.