Properly, Flash is another tier in the storage hierarchy. Faster than disk in important measures, but more expensive. Slower than RAM, but cheaper, lower pow

Properly, Flash is another tier in the storage hierarchy. Faster than disk in important measures, but more expensive. Slower than RAM, but cheaper, lower power, and durable. Unfortunately, the infrastrucure for taking easy advantage of Flash isn't very mature yet. SSDs make flash act like a hard disk, but OSs and applications generally don't allow transparent fined-grained data management to take advantage of it. I guess ZFS is best in this regard, since it can migrate hot data to fast devices, but I haven't seen good options on Linux, and I haven't seen hardware RAID controllers that could fill the gap. In your DB, you can place tables on different devices, or use table partitioning to separate recent data from cold, but that isn't as easy as adding more RAM. Ultimately though, cost effectiveness really depends on your workload. MySpace is apparently finding it cost effective to use SSDs for their cache tier. A few months back James Hamilton linked to a study that suggested that for some common workloads, the case for SSDs had to include both price/performance and performance/watt, since it didn't pencil out for each individually. That'll change though, as prices drop and performance improves.