@Ian Michael Gumby & Annihilator

Well thanks for that, but you hardly added anything to what I said and I was aware of those issues. Oracle for one can spot full table scans and avoid populating the cache with unwanted data, but that wasn't foolproof (and it's changed again in Oracle). No caching algorithm is perfect - they can only forecast the future from past behaviour.

I was also fully aware of hybrid hard drives and that they've not provided the expected benefit. However, that is surely just because of the state of technology. I'm no fan of requiring motherboard and operating system support for such things - they really ought to be transparent, much as the cache in the controller will be. Whilst the former might produce some improvements through optimising for the peculiarities of particular environments, it's usually a technological dead end as it gets locked into your particular server hardware. Much better to be able to swap out with a fast I/O device.

The particular issue here was over the special characteristics of limited write cycles on SSD. Now I've never much worried about it on "true" write activity, as most I/O on most systems is read (there are exceptions to that where state information is updated and where reads are readily serverd from cache in a DB). However, if this device really does turn a whole bunch of what would have been reads into writes to the SSD then it could wear out faster. I would always assume that the things would be fail safe, it's really the cost implications.