Tonight at Oracle OpenWorld, Oracle CEO Larry Ellison announced the latest Exadata release: X3. It sports a refreshed hardware, including a big expansion in RAM and flash storage capacity: 4TB and 22TB respectively on a full rack. That works out to 512
mbGB RAM on each database server and a bit over 1.5TB of flash on each storage server.
It’s being promoted as a hierarchical caching strategy, Exadata X3H2M2, where the most-frequently-used 4TB of data is in the database buffer cache, the next 22TB is in flash, and the least-used sits on disk.
Larry continued to explain how this approach was much better than “flash disk” storage because it could intelligently adapt to data usage patterns. This is where I disagree: There are pros and cons to both approaches.
A flash cache approach, as in Exadata X3H2M2 (Yes, it’s a mouthful. Will George Lucas sue for stealing his robot branding?) has the advantage of not requiring data redundancy: Since the master copy of the data is on a set of redundant disks, the caches don’t need the same level of redundancy, which makes for a more efficient use of flash capacity.
That having been said, flash caching implies that the entire dataset will not fit in cache and that requests for data not in cache must still go to disk, with the associated performance penalties. And depending on usage patterns (ad-hoc querying being one of the worst offenders), this uncached data access volume could be significant.