Recently, nothings askedan interesting question in a comment here :

[...] why, under the hood, deep down, is something like memcachednecessary? Why isn't the SQL server's cache as effective?

I think that one of the reasons for this is that highly aggressivecaching in an SQL server is much harder than it looks.

In database servers, caches have to be transparent ;non-transparent caches would implicitly break ACID guarantees. Now contemplate theproblem of completely correct cache invalidation that

One of the divisions in caching is between what I will call transparentand non-transparent caches. Transparent caches are ones where the onlything you are supposed to notice is faster speed; non-transparent cachesrequire you to manage them explicitly, especially cache invalidation.Operating system disk caches are an example of transparent caches, atleast in theory.

(At some level every cache is non-transparent, because it has to bemanaged by someone's code. So this is just a question of how a cache

It's not that complex software systems are inherently fragile for theusual reason, because they have more places and pieces to go wrongthan simple systems do; unlike physical machines, computer softwarehas no mechanical wear and thus doesn't just break on its own (barringintrinsic flaws). Living in a digital world, computer software thatworks keeps working forever until something changes.

The real problem with complex systems is that it's very hard for peopleto keep track of all of the interrelationships, and thus

Using DHCP to assign IP addresses is pretty popular in environments withlots of machines. You'd think that student labs, full of generic machines,would thus be a great environment for DHCP, but actually I disagree; Ibelieve that (normal) DHCP is not a great match for a lab environment.

The problem with DHCP is that it ties the IP address to the wrongthing. In a lab environment you don't want a machine's IP addressto be tied to its hardware; you want