The most convincing argument I've seen so far for why RAM-focused databases are the future. Summary: Disk I/O can't keep up with CPU architectures based on exploding numbers of cores.
This is an important document to reference for anyone converting between different BI reporting tools in the SAP ecosystem. It's also quite an interesting enumeration of the functionality you might expect to see in an OLAP interface.
More from Eric Vallo on semantic layers.
Post by Jamie Oswald discussing the multiple semantic layers SAP is currently supporting.