Spinning drives are so much slower than physical ram, I like the idea of having an intermediary (compressed ram (compcache?), SSD, etc) that I don't manage, but simply install and forget about. Then the best uses (tmp, startup, cache) occur without configuration. I could see room for recommending both since SSD is fast permanent storage while compcache may benefit large data processing.
Yes. I manage several MySQL database servers for a large forum (~360k members, thousands online at once, ~1.5m posts all within the past 9 months) and a service that is incredibly database intense. What we're doing right now is we have a read-only slave that is clocked super high and has a reasonable amount of cores (hex-core hyperthreaded I believe) for low-latency transactions and a read-write master with 32 physical cores. They both have SSDs.
To put it in perspective, APC (A php opcode caching module) reduced our PHP load on the PHP servers by about 3/4. That's just PHP. If this FlashCache kernel module has a reasonable impact I'm very very interested in compiling into the database servers because it would save quite a bit of money as we wouldn't have to get another 32-core database server and organize real database clustering so soon. The reason I can't just do it is because it's the database master that would probably benefit the most from it, and that can't be taken offline anytime but the middle of the morning and for a very short period of time.
If anybody hosting very large websites is interested in how my boss and I deal with attacks (*very* large HTTP floods, bandwidth floods), we use Varnish's inline C support to determine when a cllient is not a user by doing stuff like honeypotting to see if a certain IP makes an impossible amount of connections, whether a client loads a certain page for an infinite amount of time, whether a client switches useragents more than a reasonable amount of times, etc. and if it finds that a user is not a user it gives them an Error 418 (I'm a teapot lolololol but that can only be seen from an application) and if they continue to violate the rule they get blackhole'd. We have not had a successful HTTP flood since that was put in place.