Hybrid hard drives take me back to the Vista launch. We're finally getting around to some solutions but not on a single drive though. It feels very stillborn even today.
No, I'm talking about a hybrid inside the same form factor as a current 2.5" drive. Not using two drives. It would look to the OS and device driver as one drive. Just like the drive + cache today looks like looks like one drive. No OS changes required (just like no OS changes required for the current SSD drives plug and form factor compatible SSD drives. )
Currently, sometimes you read/write off the platters and much fewer times out of the small amount of memory on the disk. As memory gets cheaper/denser you can have a larger cache. The amount of memory can use is somewhat capped by the fact that memory GB costs much more than disk platter GB.
Flash is lower priced than RAM. So if the density goes up but the price doesn't as much you can perhaps use Flash memory to cache a bigger fraction of the platter GB space. Even more true if decide that don't need to push the increase in space on the platter as much (go with a more mature/cheaper platter tech.) and put a 1.5" platter in the package and use the extra space for Flash (cheaper energy wise to make 1.5" spin than it is to make 2.5" ones. )
Vista tried to do something that preselected what was to be pre-cached at boot or restart. And then tried to speed up a fixed situation.
This would be a more dynamic caching approach that would work with the normal disk read/write request flow. What you end up with is something that got better average speeds than what rotational drives get now but slower than the pure SSD approach. (i.e., still under 1.5 Gb/s ); at least for reads.
I'm not sure there has been a vendor that was good at both flash and platters to put something like together. Vista tried too soon. The synergies that Sun is pulling off with ZIL/ZFS caching mix between drives and flash are just now showing results with the latest version of flash memory densities and prices. There were no 1GB iPod shuffles when folks were doing that Vista stuff.
For a concrete example. The newer Hitachi Deskstar E7K1000 with 1 TB of memory only has a 32 MB cache. That is a two orders of magnitude difference between cache size and platter size. What if that was just one order? A 1 GB cache for a 1 TB disk. Or a 1 GB cache for a 300 GB disk.
Your cache hit rate is bound to go dramatically up (especially if can do some smarter prefetching from the disk). That is not what the Vista experiments were trying to do.
The rat race has been to give folks bigger and bigger disks. If you get to the point where the disks are "big enough" for most folks than can focus more attention at getting them that bounded amount of data to them faster.
I also think there are lots of motivators in the market that will keep flash disk prices higher than rotational ones for more than a few years into the future.
The bigger change for SSD drives will be when stop making them look "normal drives". A SSD drive that "plugged into" the PCI slot would likely could be made more space efficient so it looked more like a DIMM slot.
That's an OS/driver change though.
